zirconia nanocomposites—Comparison of its photocatalytic activity towards various organic pollutants

Accepted Manuscript Title: Synthesis and Characterization of Keggin-type polyoxometalate/zirconia nanocomposites—Comparison of its Photocatalytic activity towards various organic pollutants Authors: S. Sampurnam, S. Muthamizh, T. Dhanasekaran, D. Latha, A. Padmanaban, P. Selvam, A. Stephen, V. Narayanan PII: DOI: Reference:

S1010-6030(18)30911-0 https://doi.org/10.1016/j.jphotochem.2018.10.031 JPC 11547

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

30-6-2018 30-9-2018 18-10-2018

Please cite this article as: S. S, S. M, T. D, D. L, A. P, P. S, A. S, V. N, Synthesis and Characterization of Keggin-type polyoxometalate/zirconia nanocomposites—Comparison of its Photocatalytic activity towards various organic pollutants, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.10.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and Characterization of Keggin-type polyoxometalate/zirconia nanocomposites -

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comparison of its Photocatalytic activity towards various organic pollutants S. Sampurnama, S. Muthamizha, T. Dhanasekarana, D. Lathaa, A. Padmanabana, P. Selvamb, A. Stephenc, V. Narayanana,* a

Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai-600 025, India

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Department of Chemistry & NCCR, Indian Institute of Technology, Madras, Chennai-600 036, India

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Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai-600 025, India

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*Corresponding Author E-mail: [email protected]

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Graphical abstract

Highlights

The H3PW12O40/ZrO2 NPs were prepared by wet impregnation method. H3PW12O40 doping with zirconia enhances the photocatalytic activity for organic pollutants. The prepared catalysts were characterized by XRD, FTIR, Raman, DRS-UV, XPS, BET, HRSEM, HRTEM. The composite have enhanced photocatalytic activity due to the reduced band gap shown by Tauc’s plot.

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Efficiency Degradation, stability and comparision of H3PW12O40/ZrO2 NPs under UV-Visible light.

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Abstract

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Heteropolyacid supported ZrO2 nanocomposites has been prepared through wet impregnation

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method with the intention to improve the photodegradation performance under UV light. The

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material consist of mixed monoclinic and cubic ZrO2 phase and orthorhombic Keggin type 12phosphotungstic acid(PTA) with different ratios. The phase structure, chemical composition and

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oxidation state of the elements tungsten (W), zirconia (Zr) were characterized by X-ray

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diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). In addition, Fourier transform

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infrared spectroscopy (FT-IR), Raman spectroscopy, Diffuse Reflectance Ultraviolet–Visible Spectroscopy (DRS-UV–Vis) were carried out to confirm the presence of functional group and

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optical absorption properties. Surface area of the synthesized composite was analyzed by BET analysis and morphology was confirmed using high resolution scanning electron microscopy

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(HR-SEM) and high resolution transmission electron microscopy (HR-TEM) and the results reveal that PTA is chemically bonded to zirconia support which confirms that the sample was synthesized successfully. The heterogeneous photocatalytic activity of the composites were studied by using aqueous solution of various organic pollutants such as methylene blue (MB),

methyl orange (MO), rhodamine B (Rh-B), crystal violet (CV), bromo cresol green (BCG) dyes, 4- Nitrophenol and 2,4- Dichlorophenoxy acetic acid under UV light illumination. It is found that each dye needs different specific ratio of nanocomposite to degrade efficiently at a given

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time interval. Keywords: Phosphotungstic acid, Zirconia, Photocatalyst, 4- Nitrophenol, 2,4- Dichlorophenoxy acetic acid. 1. Introduction:

In recent years, industrial development has raised serious environmental problem in which

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contamination of water is a major issue, particularly textile waste water contains various

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complex constituents in which dyes are considered as the most dominating source of water

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contamination[1,2]. These dyes have high molecular weight, complex structure and low

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biodegradability [3,4]. Besides, these dyes are also mutagenic and carcinogenic[5]. The presence of dyes along with other contaminants not only hinders the biological treatment process but also

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induces allergic reactions [6,7]. Various methods have been reported for the removal of dyes

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such as adsorption, chemical and biological oxidation, coagulation and flocculation, ion

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exchange, electrochemical oxidation, and membrane separation. But these methods have drawbacks like expensive, time-consuming and leads to generation of secondary wastes[8].

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Photocatalysis is an effective environmental remediation using renewable solar energy and heterogeneous photocatalysis using semiconductors are one of the advanced oxidation processes

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(AOPs) using UV or solar light illumination[9, 10].

Heteropolyacids are polyoxometalates having attractive metal-oxygen clusters of the early transition metals (V, Nb, Mo, Ta, W) in their highest oxidation state. Among polyoxometalates (POMs), Keggin-type heteropolyanions comprised of [XM12O40]n- (where X= heteroatom as

P5+, As5+, Si4+, Ge4+; M= Metal as Mo6+, W6+, n= overall cluster charge) posses most attention, mainly because of their strong Brønsted acidity nature, high thermal stability and commercial availability[11,12]. Also without undergoing structural changes they undergo stepwise Even POM exhibits UV-light photocatalytic activity,

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multielectron redox process[13,14].

disadvantage lies in its low surface area and difficulty in separation from reaction mixture[15]. Recently these disadvantages are overcome by immobilizing POMs on solid supports such as silica, zeolite, activated carbons, mesoporous molecular sieves, layered double hydroxide and TiO2[16,17,18]. Research shows that immobilizing POMs on solid supports can conduct an

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effective photocatalytic oxidation reaction than POMs alone[19]. It has been found that among

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POMs, Keggin-type heteropolyacid particularly 12-phosphotungstic acid (H3PW12O40) show

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good photocatalytic degradation of dyes [20, 21]. When incident photon strike the POM material

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with energy greater than or equal to their band gap energy (Eg), electrons are excited from valence band (VB) to the conduction band (CB) generating electron-hole pairs which react with

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O2 and H2O respectively producing O2.- and OH. radicals responsible for advance oxidation

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process of the organic matter[22,23]. On the other hand there are only few works based on POM-

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Zirconia in the literature and their photocatalytic performance have been rarely investigated. If POM is impregnated on surfaces with strong basic character such as Al2O3, MgO it show

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moderately low chemical stability and leaches in polar solvents when supported on weak acid and neutral supports such as SiO2[24]. Hence we have choosen metal oxide zirconia as support

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for POM also synthesized a series of PTA/ZrO2 and investigated the comparative study over five different dyes and found each dye has specific ratio for better photocatalytic activity. Zirconia(ZrO2) is an n-type semiconductor material possessing optical and electrical properties, strong mechanical strength, thermal stability as well as acid-base and redox

capabilities[15,25]. Eventhough, TiO2 is second generation catalyst support, it is reducible under the reducing atmosphere or the reduced pressure where as ZrO2 is stable under those conditions as well as during photo irradiation[26]. ZrO2 can accommodate POM anions via relative strong POMs in liquid phase reactions and

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chemical bonds, these interactions avoid leaching of

preserve the primary POM structure [27-33]. ZrO2 has wide band gap value (Eg) and high negative value of the conduction band potential, which considered zirconia has photocatalyst in different chemical reactions, also depending on preparation methods the band gap energy varies between 3.25 and 5.1 eV [34].

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2. Material and Methods

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2.1. Materials

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Phosphotungstic acid (99%, Sigma Aldrich), zirconium oxychloride(ZrOCl2)(99.5%, SRL), ammonia (Merck) used in this work were of analytical reagent grade. Methylene blue,

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Rhodamine B, Methyl orange, Crystal Violet, Bromocresol green dyes were obtained from

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Qualigens. All the chemicals were used as received. Experiments were carried out using double

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distilled water and ethanol (99.9 %, Sigma-Aldrich) as solvent.

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2.2. Synthesis of Phosphotungstic acid–Zirconia nanocomposite Zirconia was prepared by dropwise addition of 17 ml (28-30%) aqueous ammonia, to 0.5 M

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(16 g) zirconyl chloride solution under stirring for 4 h and adjusted to the final pH of 10. The precipitate was filtered, washed with water and silver nitrate test was done with the filtrate solution to confirm the precipitate was free from chloride ions. The hydrous zirconia thus obtained was dried at 393K for 12 h, powdered well, and then calcined at 973K for 3 h.

PTA/ZrO2 nanocomposites was chemically synthesized by wet impregnation method, PTA (1,2,3, wt. %) is dissolved in 150 ml of ethanol, each in separate round bottomed flask fitted with refluxing unit and then ZrO2 was added so that PTA to zirconia ratio was kept as 1:1, 1:2, 1:3,

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2:1 and 3:1 w/w ratios and the suspension was stirred for 12 hours at room temperature, dried at 373K for 24 hrs in air, finally calcined at 523K for 2 hours. The obtained nanocomposites were assigned as PTA/ZR11, PTA/ZR12, PTA/ZR13, PTA/ZR21, PTA/ZR31 respectively. The step

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by step synthetic approach of PTA/ZrO2 is schematically illustrated in Fig. 1.

Fig. 1. Scheme representation for the synthesis of PTA-ZrO2 nanocomposites.

2.3. Characterization The X-ray diffraction patterns were recorded using Rich Siefert 3000 diffractometer with Cu Kα1, radiation (λ = 1.5406 Å) to determine the phase purity and structure of the as-

synthesized samples. XPS measurement was performed using MULTILAB 2000 Base system, Thermo Scientific. DRS UV-Vis absorption spectrum was recorded in the range of 200–800 nm using a Perkin Elmer lambda650 spectrophotometer. FT-IR spectroscopy was measured using

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Perkin-Elmer instrument. Raman spectroscopy was measured using laser Raman microscope, BRUKER RFS 27 instrument. BET analysis performed using Micromeritics ASAP 2030 instrument. The morphology and size of the samples were analysed by HRSEM and HRTEM using FEI Quanta FEG 200 High resolution scanning electron microscope and JEOL 3010 High

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resolution transmission electron microscopy respectively.

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2.4. UV light Photocatalytic activity

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The photocatalytic activity of the nanocomposite was tested using the organic dyes such as

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methylene blue, rhodamin B, methyl orange, bromocresol green, and crystal violet. 25mg of the

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solid photocatalyst was suspended in 100 mL of 1 × 10-5 M aqueous dye solution and then

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sonicated for 10min to disperse the catalyst homogeneously in to the dye solutions. To attain adsorption-desorption equilibrium the suspension is stirred in the dark for 30 min, then exposed

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to UV light illumination under constant stirring and withdrawn the sample after every 15 min interval, centrifuged and filtered to remove photocatalyst. The absorbance of the dyes was

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analysed by UV-Vis spectrophotometer (3600, Shimadzu).

3. Results and discussion:

3.1. Structural, optical and morphological properties Fig. 2A shows the XRD pattern of PTA, which exhibits the diffraction peaks(2ϴ) at 6.8º, 8.5º, 10.9º, 20.2º, 21.8º, 26.5º, 30.7º, 33.4º, 48.8º, 61.6º corresponding to (010), (200), (210),

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(313), (420), (610), (334), (721), (429), (964) planes of the orthorhombic H3PW12O40 (JCPDS file No. 50-0655) respectively. Fig. 2B shows the XRD pattern of the zirconia shows welldefined diffraction peaks(2ϴ) corresponds to the monoclinic phase of ZrO2 at 17.5º, 24.3º, 28.3º, 31.5º, 34.3º, 40.7º, 50.3º, 58.2º, 63º, 65.9º attributes to (100), (110), (-111), (111), (020), (-211), (220), (-222), (311), (-231) crystal planes with the JCPDS file No. 65-1022 and also the

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diffraction peaks(2ϴ) at 30.4º, 35.6º, 50.7º, 60.1º correspond to the (111), (200), (220), (311)

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planes of cubic zirconia and the obtained pattern well matches with the (JCPDS file No. 65-

Fig. 2C shows XRD patterns of all the nanocomposites (PTA/ZR13, PTA/ZR12,

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formed.

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0461) respectively. Hence mixed monoclinic and cubic crystalline phases of ZrO2 support was

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PTA/ZR11, PTA/ZR21, PTA/ZR31) are similar and no peak has been observed for PTA which

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denotes PTA is homogeneously dispersed into the zirconia support which will improve the catalytic activity of the nanocomposites. XRD pattern of nanocomposite shows more intense and

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sharp diffraction peaks due to increase in crystallization to form mostly monoclinic ZrO2 and also crystallite size increases with increase in calcination temperature[35]. In comparision, XRD

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pattern of nanocomposites before calcinations at 523 K is shown in Fig. S1. The average

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crystallite size (d) of the samples was calculated using the Scherrer formula, 𝑑 = 0.9𝜆/𝛽 cos 𝜃

where , 𝜆 is the wavelength of X-rays used, 𝛽 is the full width at the half-maximum peak value and 𝜃 is the peak position. The average crystallite size calculated for PTA/ZR 31, PTA/ZR 21,

PTA/ZR 11, PTA/ZR 12 and PTA/ZR 13 was found to be 25 nm, 24 nm, 23 nm, 18 nm, 20nm

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respectively.

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Fig. 2. XRD pattern of (A) PTA, (B) Zirconia, (C) XRD pattern of (a) PTA/ZR13 (b) PTA/ZR 12 (c) PTA/ZR 11 (d) PTA/ZR 21 (e) PTA/ZR 31.

XPS analysis was performed to examine the oxidation state and chemical composition of for PTA/ZR31 nanocomposite. Fig. 3A shows the XPS survey spectrum which confirms only the presence of Zr, W, P, O elements in sample. Fig. 3B shows the deconvoluted XPS spectra of Zr

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3d doublet shows peak with binding energy in the range of 182.43 to 184.74 eV attributes to the existence of Zr4+ species[36,37,34]. It is reported that binding energy of Zr4+ in pure zirconia is around 182.6 eV . The BE value of Zr 3d5/2 PTA/ZR31 is observed at 182.43 eV and Zr 3d3/2 at

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184.74 eV and shifted peaks also observed at 183.31 eV and 185.87 ev this is due Zr(IV) bonded

SC RI PT U N A M D TE EP CC A Fig. 3. (A) Survey spectrum, (B) Zr 3d, (C) W 4f, (D) P 2p and (E) O 1s XPS core level spectra of PTA/ZR31 nanocomposite.

to more electron attractive species and reaction of zirconia with phosphotungstic acid which is related to strong polarization of Zr-O bond [36,38,45 ]. Fig. 3C shows the intense signals at 36.22 and 38.21 eV are attributed to the W4f7/2 and W4f5/2 for W+6 in its high oxidation state

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[39,40]. The presence of high oxidation valence state of metal in composite presents excellent oxidizability, which is important in oxidative and catalytic process [40]. The Binding energy at 32.2 and 30.76 eV is attributed to W4f core level for WO2 species and W in +4 oxidation state [41 ,42 ]. Fig. 3D shows P 2p core level spectrum with binding energy value at 134.65 eV confirms the presence of phosphorous as phosphate[43,44]. Fig. 3E shows two distinguishable

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peaks for oxygen centered at 530.55 eV and 531.85 eV corresponds to lattice oxygen and non

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lattice oxygen respectively, the most intense peak at 530.55eV corresponds to lattice oxygen

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species (O2-) in WO3 (W-O) bond and the smaller peak at 531.85 eV is associated with (O2-) of

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OH- in the oxygen deficient vicinities in WO3, where O2- species bonded with Zr4+ [Zr-O] [

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39,45,46].

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Information regarding the binding of PTA molecules to ZrO2 was obtained using FT-IR spectroscopy. Fig. 4A shows FT-IR spectra of pure PTA exhibits the absorption bands at 1084

at 1084 cm

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cm- 1, 989 cm-1, 896 cm-1, 790 cm-1, 597cm -1, 519 cm-1 ranges from 700 to 1100 cm -1. The peak -1

attributed to P-O bond vibration, the peak at 989 cm-1 assigned to W=O bond

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vibration and the two peaks at 896 and 790 cm-1 corresponds to W-Oe-W and W-Oc-W bridge

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bonds respectively[47]. These absorption bands has been shifted to higher wave number in the composites at 1116cm-1, 1076cm-1, 990cm-1, 934cm-1, 894cm-1 and the band at 519cm-1 are masked by the broad band of the support [48]. Fig. 4B shows the FT-IR spectrum of zirconia sample calcined at 700 ̊C, in which absorption peak at 750 cm-1 corresponds to monoclinic zirconia and the sharp band at 520cm

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is attributed to Zr-O-Zr bond [49,50]. Fig. 4C (a)-(g)

shows the FT-IR spectra of the nanocomposites PTA, ZrO2, PTA/ZR31, PTA/ZR21, PTA/ZR11, PTA/ZR12 and PTA/ZR13.

Fig. 4D shows FT-IR spectrum of PTA (P-W) zone of the

composites the absorption bands coincides with the characteristic bands of PTA eventhough

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there is overlap with the ZrO2 support which confirms the presence of [PW12O40]3- anion in all the calcined samples, also the decrease in the intensity of bands and shift in the IR peak positions confirm the interaction between the PTA and ZrO2 support[47]. Fig. 4E shows the zirconia (ZR)

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zone of all the composite which confirms the presence of zirconia in composites.

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Fig. 4. (A) FT-IR spectra of PTA, (B) Zirconia, (C) FT-IR spectra of (a) PTA (b) ZrO2 (c) PTA/ZR 31, (d)PTA/ZR 21, (e) PTA/ZR 11, (f) PTA/ZR 12, (g) PTA/ZR 13 nanocomposites (D) P-W zones of

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(a)PTA/ZR 31, (b)PTA/ZR 21, (c) PTA/ZR 11, (d) PTA/ZR 12, (e) PTA/ZR 13 nanocomposites, (E)

zirconia zone of (a) Zirconia (b) PTA/ZR 31, (c)PTA/ZR 21, (d) PTA/ZR 11, (e) PTA/ZR 12, (f) PTA/ZR 13

Fig. 5A shows the Raman spectrum of PTA represents three bands at 1007cm -1, 989 cm-1 and 931cm-1 are assigned to stretching vibrations of P-O bond of PO4 sites, W=Ot and W-Oc/eW bonds respectively[51,31]. Fig. 5B displays intense Raman bands of ZrO2 at 100, 182, 218, dominant

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304, 338, 380, 475, 501,536, 558, 635 cm-1 these bands are characteristic of

monoclinic zirconia and cubic zirconia. Hence Raman spectra is quiet consistent with the XRD results of the material [52, 35]. Fig. 5C shows the Raman spectra of the nanocomposites (P-W zone) the bands corresponding to PTA is present, which conforms the presence of PTA in all the nanocomposites. There was red shift in the bands located at 989 cm

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, 931 cm

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of the

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composite compared to PTA which denotes weakening of W=O bonds, due to the hydrogen

band at

990 cm-1 and at 970 cm-1 corresponding

to symmetric and

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PTA/ZR13 shows

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interactions with surface Zr–OH groups[53]. The catalyst PTA/ZR31, PTA/ZR 11, PTA/ZR12,

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asymmetric W=O stretching vibrations and the band located at 825 cm-1 were indexed to the WO-Zr vibrations[54]. The composite PTA/ZR 21 shows only broad band at 995 cm-1 which

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indicates the breakage W=O double bond and the formation of W-O-Zr bond[53]. Fig. 5D shows

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Raman spectra (zirconia zones) of the nanocomposites and confirms the existence of zirconia,

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which is consistence with the FTIR results.

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Fig. 5. Raman spectra of (A) PTA, (B) Zirconia, (C) P-W zones of (a)PTA/ZR 31, (b)PTA/ZR21, (c)PTA/ZR11, (d) PTA/ZR12, (e)PTA/ZR13 and (D) ZR zones (a) Zirconia, (b) PTA/ZR 31,

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(c) PTA/ZR21, (d) PTA/ZR11, (e)PTA/ZR12, (f) PTA/ZR13 of nanocomposites

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Fig. 6A shows the Diffuse reflectance spectrum of PTA, which exhibits two bands at 255 and 354 nm corresponding to the transitions Ot _W and Ob,c_W ligand metal charge transfer (LMCT)

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of Keggin type polyoxoanion structure [32]. Fig. 6B shows the UV-Vis absorption spectra of zirconia. Fig. 6(C-E) shows the DRS UV-Vis spectrum of the nanocomposites. Fig. 6(C & D) shows spectra of PTA/ZR21, PTA/ZR11, PTA/ZR13, PTA/ZR12 nanocomposites with absorptions bands around 210 and 290 nm are assigned to charge-transfer from O2- ion to W6+ ion in Keggin units at W=O and W–O–W bonds and these bands are shifted to 225 and 260 nm

in the spectra of PTA/ZR31 (Fig. 6(E))[55]. The absorption band at 255 and 227 nm assigned to O-W CT(charge transfer) of the PTA and O- Zr CT of ZrO2 disappears and broad band appears around 260 to 390 nm in the nanocomposite which is red shifted compared to the precursors[56,

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9]. This band is tailed into UV region and disappeared around 400 nm which attributes to the outstanding photocatalytic performance of the nanocomposite in the UV region. The Band gap (Eg) was determined by using Tauc’s relationship: (∝ ℎ𝑣)1/𝑛 = 𝐴(ℎ𝑣 − 𝐸𝑔) , Where ∝ is the absorption coefficient, A is proportional constant, h is Planck’s constant, 𝑣 is the frequency of vibration, Eg is the band gap, n = 2 (for direct band gap) or n=1/2 (for indirect band gap). The

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obtained band gap (Eg) value of PTA is 2.9 eV which is in agreement with Saeed Farhadi et

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al.[57] and for zirconia it has been already reported as 5.0 eV which is close to the band gap (Eg)

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value ca. 4.72 eV [58], whereas the composites show band gap value PTA/ZR31 (2.7eV),

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PTA/ZR21 (2.6 eV), PTA/ZR11 (2.3 eV), PTA/ZR12 (2.4 eV), PTA/ZR13(2.5 eV) respectively which is low when compared to the starting PTA and zirconia which will enhance the

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photocatalytic activity of the nanocomposites.

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Bet surface area and pore size distribution The nitrogen adsorption–desorption isotherms and the corresponding BJH pore size

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distribution curves for ZrO2 and its composites are shown in Fig. 7 (A-D). The calculated Bet specific surface area, pore size and pore volume are summarized in Table. 1

SBet(m2g-1)

VP(cm3g-1)

DBJH(nm)

ZrO2

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32.2

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Sample

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The ZrO2 support calcined at 973K exhibits type IV adsorption isotherm with the hysteresis

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loop appearing at a relative pressure (p/p0) between 0.83 to 1. After the impregnation of PTA

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into the ZrO2 support , all the catalyst exhibit type IV adsorption isotherm[59]. The BET surface

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area increases and pore volume decreases in PTA/ZR31 and PTA/ZR13 catalyst, pore diameter increases in PTA/ZR31 and PTA/ZR11 compared to ZrO2 support. In PTA/ZR11 composite the

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pore volume and BET surface area decreased due to blockage of the pores by PTA.

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Fig. 7. N2 adsorption desorption isotherm and pore size distribution curves(inset) of (a) Zirconia

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(b) PTA/ZR11 (c) PTA/ZR31 (d) PTA/ZR13

Fig. 8. shows HRSEM images of (a) PTA, (b) ZR, (c) PTA/ZR13 and (d) PTA/ZR31

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nanocomposites . The HRSEM analysis confirm the shape and size of the particles. Fig. 8a shows plate like morphology of PTA with aggregation of spherical particles with the size 34 nm

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to 50 nm. Fig. 8b shows spherical particles of zirconia with the particle size ranges from 36 to 41nm . The images obtained for composites PTA/ZR13 and PTA/ZR31 is shown in Fig. 8 (c & d) shows only presence of spherical particles which confirms the presence of homogeneous dispersion of PTA in zirconia. Further, the EDAX and Elemental mapping (Fig. 9a-d) of the

composite confirms the presence of W, ZrO2, O and trace amount of phosphorus reveals the composite is pure without any impurities. Further the composite are subjected to HRTEM analysis to confirm the morphology. Fig. 10(A-D) shows the HRTEM images taken at different

nanoparticles.

which confirms the formation of spherical

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magnifications for the composite (PTA/ZR31)

The fringe pattern of the grains is shown in Fig. 10E. The d-spacing values

between two adjacent lattice planes of a crystallite were calculated to be 0.33nm corresponds to the (610) plane of PTA and 0.32nm corresponds to the (-111) plane of the monoclinic zirconia are marked in the figure respectively. The obtained plane correlate well to planes observed for

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orthorhombic PTA (JCPDS No. 50-0655) and monoclinic zirconia (JCPDS No. 65-1022)

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respectively. The planes are well correlated with XRD results. Fig. 10F shows the selected area

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electron diffraction (SAED) pattern of PTA/ZR31 nanoparticles which clearly indicates the high

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crystalline nature of the composite as observed from the XRD analysis. The results from XRD, HRTEM, SAED correlate well and confirm the presence of PTA homogeneously dispersed with

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zirconia matrix.

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Fig. 8. HR SEM images of (a) PTA (b) ZrO2 (c) PTA /ZR13 (d) PTA/ZR31 and the respective EDAX

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(b)

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(d)

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(a)

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spectrum of the composites.

Fig. 9. Elemental mapping (a) Mixed phase (b) ZrO2 (c) W (d) O of the composite PTA/ZR31.

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Fig. 10. (A-D) HRTEM images (E) Fringe pattern and (F) SAED pattern of PTA/ZR31 nanocomposite.

3.2. UV light Photocatalytic activity The photocatalytic catalytic activity of the PTA/ZR31, PTA/ZR21, PTA/ZR11, PTA/ZR12 and PTA/ZR13 nanocomposite was tested using various organic dyes methylene blue, rhodamin

B, methyl orange, bromocresol green, crystal violet. It has been already reported that ZrO2 and PTA individually exhibits lower photocatalytic activity than the composites [9]. 25mg of the solid photocatalyst was suspended in 100 mL of 1 × 10-5 M aqueous dye solution and then

SC RI PT

sonicated for 10 min to disperse the catalyst homogeneously in to the dye solution. To attain adsorption-desorption equilibrium the suspension is stirred in the dark for 30 min, then exposed to UV light under constant stirring and the samples are withdrawn after every 10 min time interval, centrifuged and filtered to remove photocatalyst. During adsorption-desorption equilibrium stirring, the samples are withdrawn for every 5 min interval, centrifuged, filtered and

U

then absorbance recorded for dye solution using UV-Vis spectroscopy. It is found that

N

equilibrium has been reached within 30 min and adsorption is more in MB (44%) and CV (43%)

A

dyes compared to Rh-B (4%), MO (10%), BCG (8%). The results are shown in Fig. S2 (A- E).

M

UV light source is rendered by 50 W high pressure mercury lamp (HPML). The concentration of the dyes before and after light irradiation was analysed by UV-Vis spectroscopy and the

𝐴𝑜 − 𝐴 ∗ 100 𝐴𝑜

EP

𝐸=

TE

calculated using the formula,

D

degradation efficiency of the dyes were shown in Fig. 11(A-F). The degradation efficiency was

CC

Where 𝐴𝑜 is the absorbance at initial time 𝑡 = 0 min and 𝐴 is the absorbance at final time

A

interval, at 𝑡 = ∞ min. It is found that PTA/ZR31 exhibits high photocatalytic activity for degradation of methylene blue (cationic) dye and the degradation efficiency was found to be 87% in 70 min compared to other composites. In case of methyl orange (anionic) dye degrades faster with the composite PTA/ZR13 with the efficiency of 81.7% at 60 min. Rhodamin B (cationic) dye degrades quickly with PTA/ZR13 composite and the degradation efficiency was

found to be 98.3% at 80 min. Crystal violet (cationic) dye degrades fast with PTA/ZR13 composite with decolourization efficiency 89.4% at 50 min. Finally bromocresol green (anionic) degrades very fast within 20 min using the PTA/ZR13 nanocomposite with the efficieny 73.2%.

Degradation Efficiency (%) Rh-B(80 min)

MO(60 min)

PTA/ZR31

87

92

45

PTA/ZR21

76

98

63

PTA/ZR11

66

88

35

PTA/ZR12

77

98

57

PTA/ZR13

78

99

82

CV(50 min)

BCG(20 min)

75

38

68

49

60

60

70

48

89

73

N

U

MB(70 min)

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Degradation efficiency of all the composites are given in the Table. 2.

A

Table. 2. shows the photocatalytic degradation of dyes catalysed by PTA/ZR nanocomposites and their

A

CC

EP

TE

D

M

degradation efficiency

SC RI PT U N A M D TE EP CC A Fig. 11. Time dependent degradation of (A) Methylene blue, (B) Methyl orange, (C) Rhodamine B, (D) Crystal Violet, (E) Bromocresol green and (F) plot of degradation efficiency of dyes againt concentration of the samples

The apparent rate constant for degradation of dyes were determined from the expression 𝐴𝑡 ln ( ) = 𝑘𝑎𝑝𝑝 𝑡 𝐴𝑜 where kapp is the apparent rate constant (min-1), t is the irradiation time (min), 𝐴𝑜 and 𝐴𝑡 are the

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absorbance at 𝑡 = 0 min and 𝑡 = ∞ min. Fig. 12 (A-D). shows the determination of apparent rate 𝐴

constant from the slope of the plot between ln (𝐴 𝑡 ) and time (min). The 𝑘𝑎𝑝𝑝 values of PTA/ZR(3:1, 𝑜

2:1, 1:1, 1:2, 1:3) were calculated and summarized in the Table. 3. The photocatalyst performance of PTA/ZR31 is better in the case of MB dye and PTA/ZR13 in the case of MO, Rh-B, CV, BCG dyes which is evidenced from the higher 𝑘𝑎𝑝𝑝 values of .PTA/ZR31(0.0156 min-1) and PTA/ZR13( 0.0456, 0.0261, 0.0342 min-1) which shows better photocatalytic activity compared to other ratios in the particular

A

CC

EP

TE

D

M

A

N

U

dye.

𝐴

Fig. 12. Plot of ln (𝐴 ) versus degradation time (A) MB (B) MO (C) Rh-B (D) CV 𝑜

MB 0.0156 0.0054 0.0092 0.0087 0.0128

Rh-B 0.0264 0.0419 0.0256 0.0378 0.0456

MO 0.0093 0.0106 0.0069 0.0125 0.0261

CV 0.0087 0.0194 0.0154 0.0163 0.0342

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Catalyst PTA/ZR31 PTA/ZR21 PTA/ZR11 PTA/ZR12 PTA/ZR13

Table. 3. The 𝑘𝑎𝑝𝑝 (min-1) values of the UV light photocatalytic degradation of dyes over the assynthesized PTA/ZR photocatalysts.

The reaction mechanism involved during the photocatalytic degradation is schematically

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represented in Fig. 13. When the PTA/ZR nanocomposites are exposed to UV light, the surface

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hydroxyl ions and oxygen molecules of the composites interact with the dye through hydrogen

A

bonding and degradation takes place through the formation of HO• and O2•– radicals, these

M

reactive oxygen species will degrade the dye. The steps involved during photochemical reaction

D

between PTA/ZR and the dye is written as follows:

EP

O2 + e–CB → O2•–

TE

PTA-ZrO2 + hv → PTA-ZrO2 (e–CB + h+VB)

O2•– + H+ → HO2•

CC

O2•– + H2O → HO2• + OH–

HO2• + H2O → H2O2 + HO•

A

h+VB + H2O → HO• + H+ H2O2 → 2HO• HO• + O2•– + Dye → CO2 + H2O

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Fig. 13. The mechanism of photocatalytic degradation of dye over PTA/ZR photocatalyst .

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In order to further evaluate the photocatalytic activity of the nanocomposites, 4-

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Nitrophenol (4-NP) and 2,4- dichlorophenoxy acetic acid (2,4- D) is selected as other model

A

pollutants. 4- NP, 2,4- D is the most toxic organic compound and commonly used herbicide

M

respectively, degraded using PTA/ZR31 composite. All experimental conditions were same as dye degradation. Fig. 14(A-D) & 15(A-D) shows photodegradation of 4- NP and 2,4-D under

D

UV light irradiation respectively. 40 mg of photocatalyst was added to the 100 mL of 1 × 10-5 M

TE

aqueous solution of 4-NP and 2,4-D taken separately and stirred in dark for 30 min to attain

EP

adsorption-desortion equilibrium and then exposed to light irradiation under constant stirring. The reaction mixture was collected at regular interval, centrifuged, filtered to remove

CC

photocatalyst and subjected to UV-Vis analysis. The characteristic absorption band of 4-NP at (λmax:400 nm) shows decrease in absorbance during light irradiation and photocatalytic

A

decolourization efficiency was determined to be 90% in 90 min and the rate constant 𝑘𝑎𝑝𝑝 be 0.02373(min-1) . 2,4-D shows the absorbance at the characteristic wavelength (λmax: 282.5 nm) and this absorbance value decreased 85% in 120 min and the rate constant 𝑘𝑎𝑝𝑝 be 0.015(min1

).

SC RI PT U N A M D TE

Fig. 14. (A) Photocatalytic degradation of 4-NP (B) 𝐴⁄𝐴𝑜 vs time (C) ln 𝐴⁄𝐴𝑜 vs time (D)

A

CC

EP

degradation efficiency of the nanocomposite.

SC RI PT U N A M D

TE

Fig. 15. (A) Photocatalytic degradation of 2,4- D (B) 𝐴⁄𝐴𝑜 vs time (C) ln 𝐴⁄𝐴𝑜 vs time (D)

EP

degradation efficiency of the nanocomposite. 3.3. Photostability and recyclability studies

CC

Photostability and reusability is an essential aspect for the photocatalyst to reduce the

A

cost as well as to apply for commercial applications. The best performing photocatalyst, PTA/ZR31 nanocomposite for MB degradation is selected for recycle degradation experiment for three cycles, the composite was washed with ethanol to remove the absorbed dye molecules after each recycle, then washed with distilled water and dried. The dried PTA/ZR31 photocatalyst was used for degradation of MB dye, and the UV-Vis spectra for third recycle

photodegradation is shown in Fig. 16D, this spectra clearly shows the peak around 237 and 260 nm which is already present in the composite PTA/ZR31. For comparison, the UV-Vis spectra of first cycle degradation of MB is shown in Fig. S3. It is found that after 3 times recycle, the

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composite is able to degrade more than 80% of MB (Fig. 16A & B) which confirms its reusability. In addition the used sample, PTA/ZR31 are characterized after the third recycle photodegradation using XRD, HRSEM and EDAX (Fig. 16 C, E, F) and showed no obvious changes in the structure, morphology and confirms the presence of tungsten and phosphorus in the used samples respectively. These results obtained clearly concludes that the nanocomposite

U

PTA/ZR31 do not undergo any structural changes during the photocatalytic reactions and possess

N

excellent stability hence it can be practically used in prolonged period of time in wastewater

A

CC

EP

TE

D

M

A

treatment.

SC RI PT U N A M D TE EP CC A Fig. 16. (A) Reusability of the catalyst for three runs (B) Photocatalytic degradation percentage (C) XRD patterns before and after photodegradation (D) UV-Vis spectrum after the third recycle

(E) HRSEM image after third recycle (F) EDAX spectra with element ratio after third recyle photodegradation reaction of MB using PTA/ZR31 nanocomposite 3.4. ESI-MS+ analysis

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The possible intermediates formed during the photodegradation was analysed by ESI-MS+ technique. The ESI-MS+ spectrum (Fig. 17A) and the plausible degradative intermediates for the photocatalytic degradation of MB dye sample are shown in Fig. S4. The mass peaks at m/z 318, 284,

270,

166,

151,

110

corresponds

to

MB

photodegraded

by-products

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and prolonged irradiation may lead to complete mineralization of the dye molecules. The ESI-

N

MS+ analysis of photodegraded 4-NP sample (Fig. 17B) showed the absence of peak at m/z 139,

A

which is the molecular ion peak of 4-NP, this confirms the complete degradation of 4-NP. The

M

peaks at m/z 125, 110, 109, 102 corresponds to 4-NP degraded fragments. The ESI-MS+ analysis of 2,4-D (Fig. 17C) showed the absence of 2,4-D molecular ion peak at m/z 220 and the peaks at

D

163, 118, 110, 109 corresponds to 2,4-D photodegraded fragments using PTRZR31

TE

nanocomposites. The plausible intermediates formed during photodegradation for 4-NP and 2, 4-

A

CC

EP

D is shown in Fig. S5 & S6.

D

TE

EP

CC

A

SC RI PT

U

N

A

M

SC RI PT U N A M

TE

4. Conclusion

D

Fig. 17. ESI-MS+ Spectrum of photodegraded (A) MB (B) 4-NP (C) 2,4-D

EP

Keggin-type, H3PW12O40/Zirconia (PTA/ZR) composites were prepared by wet impregnation method. The composites prepared using ZrO2( the mixed monoclinic and cubic

CC

phases) as support and PTA shows reduced band gap value and improved photocatalytic activity. The homogeneous dispersion of Keggin unit in ZrO2 support was confirmed by FTIR, Raman

A

and XPS analysis. The mixed crystalline phase of ZrO2 and orthorhombic structure of PTA is confirmed from XRD analysis. Elemental analysis and surface morphology were explained using EDX, HRSEM and HRTEM analysis. The photocatalytic activity of the composites were compared using cationic dyes like methylene blue, rhodamine B, crystal violet and anionic dyes

like methyl orange, bromocresol green. The maximum photocatalytic acivity was found with PTA/ZR31, with MB and PTA/ZR13 with RhB, MO, CV, BCG dyes respectively. Other pollutants like 4-NP and 2,4- Dichlorophenoxy acetic acid were also photodegraded using

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PTA/ZR31 composite which shows excellent photoactivity towards these compounds. The obtained results reveal the specific ratio of the composite to degrade the dyes effectively from the contaminated water, due to higher surface area of the catalysts as evidenced from BET analysis. Further the method adopted to prepare the composite is simple and cost effective for solving the environmental problems.

N

U

Acknowledgments

A

The authors (SS) acknowledges the Department of Chemistry, IIT (Madras) for providing

M

XRD, FTIR, DRS-UV and SAIF (IIT Madras) for Raman, HRSEM measurements. CECRI, Karaikudi for providing XPS facility. CATERS, CLRI for providing ESI-MS+ facility.

TE

D

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