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Applied Surface Science 377 (2016) 17–22

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

novel iron-containing polyoxometalate heterogeneoushotocatalyst for efficient 4-chlorophennol degradation by H2O2 ateutral pH

ian Zhai a, Lizhong Zhang a, Xiufeng Zhao a,∗, Han Chen a, Dongju Yin a, Jianhui Li a,b,∗∗

Department of Chemistry and Applied Chemistry, Changji University, Changji 831100, ChinaNational Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamenniversity, Xiamen 361005, China

r t i c l e i n f o

rticle history:eceived 14 December 2015eceived in revised form 9 March 2016ccepted 10 March 2016vailable online 15 March 2016

eywords:olyoxometalateeterogeneous Fentonhotocatalysis

a b s t r a c t

An iron-containing polyoxometalate (FeШLysSiW) was synthesized from ferric chloride (FeIII), lysine (Lys)and silicotungstic acid (SiW), and characterized using ICP-AES, TG, FT-IR, UV–vis DRS, XRD and SEM.The chemical formula of FeШLysSiW was determined as [Fe(H2O)5(C6H14N2O2)]HSiW12O40·8H2O, withKeggin-structured SiW12O40

4− heteropolyanion and lysine moiety. As a heterogeneous catalyst, the asprepared FeШLysSiW showed good performance in the degradation of 4-chlorophenol by H2O2 in boththe dark and irradiated systems. Under the conditions of 4-chlorophenol 100 mg/L, FeШLysSiW 1.0 g/L,H2O2 20 mmol/L and pH 6.5, 4-chlorophenol could be completely degraded in ca. 40 min in the dark andca. 15 min upon irradiation. Prolonging the reaction time to 3 h, the TOC removal reached to ca. 71.3% inthe dark and ca. 98.8% under irradiation. The catalytic activity of FeШLysSiW stems from synergetic effect

4−

egradation-Chlorophenol

of ferric iron and SiW12O40 in the catalyst, corresponding to Fenton-like catalysis and photocatalysis,respectively. The enhanced degradation of 4-CP under irradiation is due to the simultaneous oxidationof 4-CP through the Fenton-like and photocatalytic processes. The high catalytic activity of FeШLysSiWis also strongly related to the chemisorption of H2O2 on FeШLysSiW surface by hydrogen bonding, whichpromotes both the Fenton-like and photocatalytic processes.

© 2016 Elsevier B.V. All rights reserved.

. Introduction

Advanced oxidation processes (AOPs), involving in situ gen-ration of hydroxyl radicals (HO•), are widely used for oxidativeegradation of organic pollutants. As one of the most importantOPs, Fenton or related oxidation processes have been intensivelytudied. Conventional homogeneous Fenton process occurring ine2+/H2O2 aqueous system produces HO• from H2O2 decomposi-ion catalyzed by Fe2+, in which organic pollutants are oxidized byhe produced HO• consequently. This approach is typically operatednder acidic pH conditions (pH 2–3); acidification and neutral-

zation are therefore usually needed respectively before and after

rocessing. Additionally, the ferrous salt added in water precipi-ates in the form of iron sludge, which needs further disposal [1–5].herefore, the development of reusable solid state catalysts with

∗ Corresponding author at: No. 77 Beijing Road, Changji 831100, Xinjiang, China.∗∗ Corresponding author at: Department of Chemistry and Applied Chemistry,hangji University, Changji 831100, China.

E-mail address: zhaoxiufeng19670@126.com (X. Zhao).

ttp://dx.doi.org/10.1016/j.apsusc.2016.03.083169-4332/© 2016 Elsevier B.V. All rights reserved.

wide working pH range for heterogeneous Fenton-like processeshas been a hot research topic. Many solid materials, such as zerovalent iron (ZVI) [6–8], iron oxides and their composites [9–14],BiFeO3 [15–17], FeVO4 [18], and iron-containing zeolites, silicasand clays [19–22] have been reported. The catalytic mechanismof these iron-containing materials is related to the production ofHO• via FeII � FeIII redox cycle on the catalyst surface (Eqs. (1)–(2)),being similar to that in homogeneous Fe2+/H2O2 or Fe3+/H2O2 sys-tem.

FeII + H2O2 → FeIII + HO− + HO• (1)

FeIII + H2O2 → FeII + H+ + HOO• (2)

However, many of these catalysts are still restricted by acidicoperating conditions and suffer from serious deactivation in con-secutive reaction cycles.

Polyoxometalates (POMs), the well-defined metal-oxygen clus-

ters with a wide variety of structures and properties, demonstratedesirable reversibility in multi-electron redox reaction, whichmake them attractive redox catalysts and photocatalysts [23,24].In POM photocatalysis, a widely accepted catalytic mechanism

1 face Science 377 (2016) 17–22

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8 Q. Zhai et al. / Applied Sur

s related to the excitation of POM under irradiation, when lightnergy is higher than or equal to the gap between the highestccupied molecular orbital (HOMO) and the lowest unoccupiedolecular orbital (LUMO) of a POM. The excited POM has strong

xidative capability to consume many organic matters directly.dditionally, the excited POM can also oxidize H2O to produceO•, which subsequently oxidizes organics. The reduced POM (i.e.OM−) is re-oxidized by O2 to close the photocatalytic redox cycle25–28]. Recent years, the use of POM in Fenton or related pro-esses has attracted increasing interest. Chio et al. [29] confirmedhat the production of Fe2+ and H2O2 from the oxidation of Fe◦

y O2 in Fe◦/O2 system is facilitated by addition of SiW12O404−

r PW12O403−, which promotes the degradation of organic pol-

utants consequently. Taghdiri et al. [30] reported the catalysis ofilicotungstic acid for hexamethylenetetramine degradation in theresence of H2O2 or Fe2+/H2O2. Notably, Lee and Sedlak [31] foundhat the formation of soluble complexes between iron ions (Fe3+

nd Fe2+) and PW12O403− in homogeneous Fe3+/H2O2 system sig-

ificantly promoted the oxidation of organic compounds in a wideH range. These previous studies imply the potential synergeticatalytic effect between iron and heteropolyanion on Fenton orelate processes. Therefore, it is highly possible that the water-nsoluble complexes containing both iron and heteropolyanion,amely iron-containing POMs, are a kind of efficient heteroge-eous Fenton-like catalysts. The complexes formed by ferric ionnd many heteropoly acid are water-soluble, however, we haveound that ferric ion coordinated by some amino acid ligandsan form insoluble iron-containing POMs with some heteropolycids, and some of these compounds show high catalytic activ-ty for heterogeneous Fenton-like processes [32–34]. To eventuallyeveal the relationship between the catalytic properties and thehemical compositions of these iron-containing POMs, it is nec-ssary to study the catalytic properties of these compounds withiverse amino acid ligands and heteropolyanions. Herein, we report

new FeIII-containing silicotungstate catalyst (FeIIILysSiW) for-chlorophenol (4-CP) degradation by H2O2 at neutral pH. The cat-lytic mechanism was also discussed.

. Materials and methods

.1. Reagents

Ferric chloride hexahydrate, lysine, silicotungstic acid, sodiumilicate, sodium tungstate and hydrogen peroxide (30%) were pro-ided by Sinopharm Chemical Reagent Beijing Co., Ltd. 4-CP wasbtained from Tianjin Guangfu Fine Chemical Industry Research

nstitute. All the reagents were of analytical grade and used withouturther purification.

.2. Synthesis and characterization of FeIIILysSiW

FeIIILysSiW was synthesized from ferric chloride (FeIII), lysineLys) and silicotungstic acid (SiW) at ambient temperature. Lysine0.15 g) and ferric chloride hexahydrate (0.27 g) were firstly addednto water (25 mL) to produce a soluble ferric lysine complex.fter being stirred for 1 h, a solution of silicotungstic acid (25 mL,.02 mol/L) was added dropwise, producing the FeIIILysSiW precip-

tate. The mixture was then sealed and stirred for 2 h. After beingged at ambient temperature for 10 h, the light brown FeIIILysSiWowder was obtained by centrifuging, washing with water, andrying in vacuo at 50 ◦C.

The synthesized FeIIILysSiW sample was characterized by induc-ively coupled plasma atomic emission spectrometry (ICP-AES,himadzu ICPS-7510), thermogravimetry (TG, Seiko TG/DTA6300),ourier translation infrared spectrometry (FT-IR, Shimadzu

Fig. 1. TG and DTG curves of FeIIILysSiW.

IRAffinity-1), UV–vis diffuse reflection spectrometry (UV–vis DRS,Shimadzu UV2550), X-ray diffraction (XRD, XD-2, Cu K ̨ radia-tion), scanning electron microscopy (SEM, Hitachi S-4800) andX-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific,ESCALAB 250Xi).

2.3. Degradation of 4-CP

The catalytic performance of FeIIILysSiW was evaluated by 4-CP degradation in the presence of H2O2. All the experiments wereseparately performed under dark and irradiated conditions. Thedegradation of 4-CP was carried out in a cylindrical quartz tubeunder magnetic stirring. A 400 W high pressure mercury lamp(�max = 365 nm) was used as the source of irradiation, which wassurrounded by a quartz jacket to allow for water-cooling. The lightflux at the liquid level was ca. 450 �W/cm2. In a typical procedure,the powdered FeIIILysSiW catalyst was firstly added into a 4-CPsolution (100 mg/L) with magnetic stirring to maintain a uniformsuspension. The suspension pH was adjusted using dilute H2SO4or NaOH solution, and then H2O2 was added. No pH control wasapplied during the process. In photo-assisted Fenton-like process,the addition of H2O2 was performed simultaneously with irradi-ation. Specimens were withdrawn at regular time intervals andanalyzed immediately after filtration through a 0.22 �m filter toremove the catalyst powder.

The concentration of 4-CP was analyzed using a high perfor-mance liquid chromatograph (HPLC, Shimadzu LC-20AD) equippedwith a C18 reverse phase column and an UV detector. The mobilephase was a mixture of methanol (65%) and 1% of acetic acid aque-ous solution (35%) at the flow rate of 0.5 mL/min. The concentrationof total organic carbon (TOC) was determined by a TOC analyzer(Shimadzu TOC-V CPH).

3. Results and discussion

3.1. Characterization of FeIIILysSiW

The chemical formula of FeIIILysSiW is determined to be[Fe(H2O)5(C6H14N2O2)]HSiW12O40·8H2O on the basis of the ICP-AES, TG and FT-IR results. Calcd for FeIIILysSiW (%): Fe 1.69, Si 0.85,and W 66.6; found by ICP-AES (%): Fe 1.72, Si 0.82, and W 66.3. TheICP-AES data for Fe, Si, and W contents in FeIIILysSiW are close to

the calculated values based on the proposed chemical formula. TGand DTG curves of FeIIILysSiW in Fig. 1 exhibits a multi-step weightloss from 30 to 506 ◦C. The 4.6% weight loss from 30 to 167 ◦C isdue to the removal of eight crystalline H2O molecules (calcd: 4.4%).

Q. Zhai et al. / Applied Surface Science 377 (2016) 17–22 19

Table 1Main relevant FT-IR data of SiW and FeIIILysSiW.

SiW FeIIILysSi FeIIILysSiWd FeIIILysSiWi Assignments

775 793 795 797 W Oc W asymmetric stretching880 881 883 883 W Ob W asymmetric stretching926 920 918 920 Si Oa asymmetric stretching982 986 986 986 W Od asymmetric stretching

1350 (s) 1350 (s) 1350 (s) C H bending1381 (w) 1383 (w) 1383 (w) Symmetric mode of COO and C N stretching1599 (vs) 1595 (vs) 1595 (vs) Bending mode of NH3

+

1630 (vs) 1630 (vs) 1630 (vs) Asymmetric mode of COO coordinated with Fe3+

d FeIIILysSiW used in the dark.i FeIIILysSiW used under irradiation.

F III III

(

Tols

AiKF9ilimitm

S4talcFwadgtF

Fig. 3. UV-vis DRS of (a) SiW and (b) FeIIILysSiW.

Fe2+ can be generated by the reaction of Fe3+ and H2O2, as indi-

ig. 2. FT-IR spectra of (a) SiW, (b) Fe LysSiW, (c) Fe LysSiW used in the dark, andd) FeIIILysSiW used under irradiation.

he 2.7% weight loss from 167 to 280 ◦C is ascribed to the removalf five coordinated H2O molecules (calcd: 2.7%). The 4.3% weight

oss in the following steps from 280 to 506 ◦C is attributed to thetepwise deconstruction of lysine moiety (calcd: 4.4%).

The FT-IR spectra of SiW and FeIIILysSiW are presented in Fig. 2.ssignments of peaks based on literature values [35–37] are shown

n Table 1. SiW shows the characteristic infrared fingerprints ofeggin-structured SiW12O40

4− at 775, 880, 926 and 982 cm−1.eIIILysSiW has similar absorption peaks at 793, 881, 920 and86 cm−1, indicating that SiW12O40

4− heteropolyanion is remainedn FeIIILysSiW. Moreover, FeIIILysSiW displays IR absorptions fromysine moiety in the range of 1300–1700 cm−1. Therefore, FT-IR datandicate the existence of SiW12O40

4− heteropolyanion and lysineoiety in FeIIILysSiW. The red and blue shifts for the four character-

zed vibrations of Keggin structure in FeIIILysSiW are resulted fromhe interaction between SiW12O40

4− heteropolyanion and lysineoiety or ferric ion.

The UV–vis DRS of SiW and FeIIILysSiW are presented in Fig. 3.iW displays a wide absorption band in the range from 200 to50 nm peaked at 210 and 265 nm, which are attributed to charge-ransfer from O2− to W6+ in Keggin-structured SiW12O40

4− at W = Ond W−O−W bonds, respectively [25]. FeIIILysSiW has the simi-ar absorption peaks at 210 and 267 nm, further supporting theonclusion that SiW12O40

4− unit exists in FeIIILysSiW. However,eШLysSiW shows an obvious red-shift of the absorption edge,hich may be beneficial for its photocatalysis because of the rel-

tively narrow band gap. The band gap of SiW was diffcult to beetermined accurately since it is hard to get the reasonable tan-

ent in the DRS spectra for SiW sample. However, it is clear fromhe DRS results that the band gap for pure SiW is quite higher thaneIIILysSiW with the calculated value of 2.76 eV.

Fig. 4. XRD patterns of (a) SiW and (b) FeIIILysSiW.

XRD patterns of SiW and FeШLysSiW are given in Fig. 4(a). SiWis highly crystalline, but FeШLysSiW has the reduced crystallinity.This is partly due to the different counter-cations in FeШLysSiW andSiW [24]. XPS characterization was performed for FeШLysSiW sam-ple. The results of curve fitting with linear background subtractionbetween 700 and 716 eV give the Fe 2p3/2 peak position for Fe3+

at 711.0 eV. It was found that Fe3+ is the predominant species and

cated in Eq (2). The SEM image of FeIIILysSiW in Fig. 5 indicates thatthe sample appears to be the irregular particles with the size of ca.50–100 nm.

20 Q. Zhai et al. / Applied Surface S

Fig. 5. SEM image of FeIIILysSiW.

Fc

3

tucnaaidgbiowdtt

toFc

freshly prepared and the used FeШLysSiW samples. Further exper-

ig. 6. Catalytic performance of FeIIILysSiW for 4-CP degradation at pH 6.5. Reactiononditions: H2O2 20 mmol/L, FeIIILysSiW 1.0 g/L, temperature 25 ± 2 ◦C.

.2. Catalytic performance of FeШLysSiW

To compare with that of ZVI and Fe3O4 reported in the litera-ures [7–9], the catalytic performance of FeШLysSiW was evaluatednder the similar conditions, i.e., 4-CP 100 mg/L, H2O2 20 mmol/L,atalyst 1.0 g/L. The initial pH value was measured to be 6.5, ando pH adjustment was performed. The results showed that the cat-lytic activity of ZVI and Fe3O4 is very weak, since they showedlmost no catalytic activity under neutral condition. Control exper-ments showed that, with H2O2 or FeШLysSiW alone in both theark and irradiated systems, the depletion of 4-CP could be negli-ible within 40 min. This indicated that the direct oxidation of 4-CPy H2O2, or photocatalytic oxidation of 4-CP by FeШLysSiW, was

nsignificant within the observed time. In contrast, in the presencef both FeШLysSiW and H2O2, rapid decrease in 4-CP concentrationas observed. 4-CP was completely disappeared in ca. 40 min in the

ark and ca. 15 min upon irradiation, separately (Fig. 6). Prolonginghe reaction time to 3 h, the TOC removal reached to ca. 71.3% inhe dark and ca. 98.8% under irradiation (inset in Fig. 6).

It was reported that, at pH ≥ 4, ZVI and Fe3O4 magnetic nanopar-icles showed almost no catalytic activity for the degradation

f 4-CP and 2,4-dichlorophenol, respectively [7,9]. In contrast,eШLysSiW has high catalytic activity at neutral pH (pH 6.5), and theatalytic performance is significantly enhanced under irradiation.

cience 377 (2016) 17–22

Therefore, FeШLysSiW is a promising heterogeneous Fenton-likecatalyst for degradation of organic pollutants.

In Fenton or related processes, HO• is considered to be related tothe oxidation of organic matters, and n-butanol is usually used as aprobe molecule for HO• detection owing to its powerful ability toscavenge HO• [9]. In FeШLysSiW/H2O2 system under both the darkand irradiated conditions, the degradation of 4-CP was consider-ably retarded when 0.3 mol/L of n-butanol was added. Therefore thedominant reactive oxidant generated in FeШLysSiW/H2O2 systemis suggested to be HO•.

The catalytic performance was affected by initial pH, H2O2 con-centration and FeШLysSiW amount. As shown in Fig. 7(a), thereaction rate was increased with the decrease in initial pH. Whenthe pH was lowered to 3.0, the time required to degrade 100% of4-CP was achieved in ca. 30 min in the dark and ca. 10 min underirradiation. On the contrary, with raising initial pH to 8 or 9, thereaction was significantly impeded both in dark and irradiatedsystems. The enhanced oxidation of 4-CP at lower pH values isattributed to the increase in oxidation potential of HO•. Moreover,the higher stability of H2O2 in acidic solution leads to less H2O2decomposition immediately to H2O and O2, which favors the pro-duction of HO• via Eq. (1) [9,35,36]. Though a lower pH values wasconductive to the catalysis of FeIIILysSiW, the reaction efficiency atthe natural pH (pH 6.5) was still considerable. Thus, the followingexperiments were carried out without initial pH adjustment.

As shown in Fig. 7b, the reaction rate decreased with loweringH2O2 concentration from 20 to 10 mmol/L, due to the reduced HO•

production with insufficient H2O2 (Eq. (1)). The reaction efficiencywas not changed significantly with increasing H2O2 concentrationfrom 20 to 30 mmol/L, attributing to scavenging of HO• by excessiveH2O2 (Eq. (3)) [8,38].

4H2O2 + HO• → HO2• + H2O (3)

The effect of FeШLysSiW amount is given in Fig. 7c. Withdecreasing FeШLysSiW amount from 1.0 to 0.4 or 0.2 g/L, the reac-tion was slowed down due to the reduced active sites of thecatalyst. However, the reaction rate did not change significantlywith increasing FeШLysSiW amount from 1.0 to 2.0 g/L, being possi-bly ascribed to the agglomeration of the catalyst particles [9,39,40].

Therefore, at the natural pH of 100 mg/L 4-CP (pH 6.5),FeШLysSiW has the highest catalytic performance under the con-ditions of H2O2 20 mmol/L and FeШLysSiW 1.0 g/L.

3.3. The stability and reuse of FeШLysSiW

The stability of a solid heterogeneous catalyst is crucial to itsreusability. For the degradation of 4-CP in FeШLysSiW/H2O2 systemat pH 6.5, FeШLysSiW may be decomposed through two possibleroutes: hydrolysis of SiW12O40

4− in FeШLysSiW [24], or degra-dation of lysine moiety in FeШLysSiW. The former route releasesFe3+, SiO3

2−and WO42− into the bulk solution; the latter releases

Fe3+ and SiW12O404− in bulk solution. Hence, the elemental con-

centrations of Fe, Si and W in solution after the catalytic reactioncan reflect the degree of FeШLysSiW decomposition. The ICP-AESdata of Fe, Si and W contents in solution are ca. 0.0085, 0.0082and 0.11 mmol/L for the dark system, and ca. 0.0091, 0.0093 and0.11 mmol/L for the irradiated system, respectively. This indicatesthat less than 3% of FeIIILysSiW decomnposed during the reac-tion. Moreover, the used FeШLysSiW was collected after reactionand characterized using FT-IR. As shown in Fig. 2 and Table 1,there are no obvious differences between the FT-IR spectra of the

iments also confirmed that there were no significant differencesin the FeШLysSiW activity after three cycles for both the dark andirradiated systems. The FeШLysSiW catalyst is therefore stable dur-

Q. Zhai et al. / Applied Surface Science 377 (2016) 17–22 21

Fig. 7. Effect of (a) initial pH, (b) H2O2 concentration, and (c) FeIIILysSiW amount on the catalytic performace of FeIIILysSiW. Excepte for the tested parameters, others fixedat pH 6.5, H2O2 20 mmol/L, FeIIILysSiW 1.0 g/L, and temperature 25 ± 2 ◦C.

iFsPmtia

3

pmpc(t(tbatyFacmuas

rs[FbaN

ng 4-CP degradation. SiW12O404− immobilized in water-insoluble

eШLysSiW is more resistant to hydrolysis than is SiW12O404− dis-

olved in water. This is similar to the reported observation thatW12O40

3− immobilized in a SiO2 matrix is stable in a neutral pHedium [41]. Also, in contrast to the fast oxidation of 4-CP, oxida-

ion of the immobilized lysine moiety in the FeШLysSiW catalysts retarded. The exact reasons for the stabilities of the SiW12O40

4−

nd lysine moiety in FeШLysSiW need further investigation.

.4. Catalytic mechanism of FeШLysSiW

To detect the heterogeneous nature of the catalytic process atH 6.5, control experiments were performed in two homogeneousixed solutions, each containing 100 mg/L of 4-CP, to investigate

ossible degradation of 4-CP in the bulk solution. One solutionontained 0.01 mmol/L Fe3+ (ferric chloride), 0.01 mmol/L SiO3

2−

sodium silicate), and 0.11 mmol/L WO42− (sodium tungstate), and

he other contained 0.01 mmol/L Fe3+ and 0.01 mmol/L SiW12O404−

silicotungstic acid). The two aqueous solutions are equivalent tohe bulk solution phase of the FeШLysSiW/H2O2 system, on theasis that 3% of the catalyst decomposes during the reaction asforementioned in section 3.3. The former solution correspondso the case in which FeШLysSiW decomposes through hydrol-sis of SiW12O40

4−, and the latter represents decomposition ofeШLysSiW through oxidation of lysine moiety. Both solutions weredjusted to pH 6.5, followed by addition of H2O2 to a final con-entration of 20 mmol/L. The changes in 4-CP concentration wereonitored. The results show that 4-CP degradation was negligible

nder both dark and irradiated conditions. The formation of HO•

nd degradation of 4-CP in the heterogeneous FeШLysSiW/H2O2ystem therefore probably occur on or near FeШLysSiW surface.

Therefore, the catalytic mechanism of FeШLysSiW is rationallyelated to redox cycle FeII/FeIII on the catalyst surface (Eqs. (1)–(2)),imilar to that of iron-based materials reported in the literature8,9,17]. However, this can not completely explain the fact that

eШLysSiW has higher catalytic activity than many reported iron-ased materials. Unlike ZVI, Fe3O4 magnetic nanoparticles, BiFeO3nd Fe2(MoO4)3, FeШLysSiW contains organic ligand lysine withH3

+ and COO− groups, which can adsorb H2O2 by hydrogen

bonding. Interestingly, Wang et al. [17] reported that the addi-tion of organic ligand ethylenediaminetetraacetic acid (EDTA), alsocontaining NH3

+ and COO− groups, into BiFeO3/H2O2 system cansignificantly enhance the oxidation of bisphenol A. They confirmedthat the hydrogen bonding between H2O2 and adsorbed EDTA onBiFeO3 surface could concentrate H2O2 on local surface and lowerthe electron density of O O bond of H2O2, being favorable to thebreakage of H2O2 to produce HO• via Eqs. (1)–(2). Therefore, it canbe conceivable that the adsorption of H2O2 on FeIIILysSiW surfaceby hydrogen bonding with NH3

+ and COO− groups of lysine moi-ety of FeIIILysSiW may be the essential reason for the high catalyticperformance of FeIIILysSiW.

We have checked the species in the solutions after degra-dation reaction by HPLC. Some kinds of intermediates, such as4-chlorocatechol, pyrocatechol and 1,2,4-benzenetriol substances,can be detected both in dark and irradiation conditions. Thereis almost no difference between the two conditions owing tothe weak signals of these intermediate substances. The enhanceddegradation of 4-CP under irradiation is related to the presenceof SiW12O40

4− in FeШLysSiW. The widely accepted photocatalyticmechanism of POM can well explain the increased degradation rateof 4-CP in FeIIILysSiW/H2O2 system. When the reaction was con-ducted under irradiation, 4-CP can not only oxidized via Fenton-likemechanism, as mentioned above, but also via POM photocatalyticmechanism as expressed Eqs. (4)–(8) [25–28].

[SiW12O404−] + h� → [SiW12O40

4−]∗ (4)

(5) [SiW12O404−]* + H2O → [SiW12O40

5−] + H+ + HO•

HO• + 4-CP → oxidizedproduct (6)

[SiW12O404−]∗ + 4-CP → [SiW12O40

5−] + oxidizedproduct (7)

[SiW12O405−] + O2 → [SiW12O40

4−] + O2− (8)

Previous studies have confirmed that, as the oxidation catalystsin solutions, POMs by themselves are not good catalysts because

re-oxidation of POM− to POM by O2 is rather slow (Eq. (8)) [24].This is why the degradation of 4-CP under irradiation with addi-tion of FeIIILysSiW alone was quite slow aforementioned in section3.2. However, it has been reported that, in the degradation of hex-

2 face S

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[catalysts for heterogeneous Fenton-like reactions—influence of Fe(II)/Fe(III)ratio on catalytic performance, J. Hazard. Mater. 241–242 (2012) 433–440.

[41] B. Yue, Y. Zhou, J. Xu, Z. Wu, X. Zhang, Y. Zou, S. Jin, Photocatalytic degradation

2 Q. Zhai et al. / Applied Sur

mine catalyzed by silicotungstic acid in the presence of H2O2 or2O2/Fe2+, the reduced SiW12O40

4− (i.e. SiW12O405−) can be re-

xidized by H2O2, producing additional HO• (Eq. (9)) [31].

SiW12O405−] + H2O2 + H+ → [SiW12O40

4−] + H2O + HO• (9)

Thus, in relative to the negligible degradation of 4-CP under irra-iation with FeIIILysSiW alone, the rapid degradation of 4-CP ineIIILysSiW/H2O2 system can be explained by the participation of2O2, particularly hydrogen bonded on the catalyst surface, in thehotocatalysis of FeIIILysSiW through Eq. (9).

. Conclusions

An iron-containing silicotungstate (FeIIILysSiW) was synthe-ized and used as the heterogeneous Fenton-like catalyst for-CP degradation. The chemical formula of FeIIILysSiW is deter-ined to be [Fe(H2O)5(C6H14N2O2)]HSiW12O40&903;8H2O, with

eggin-structured SiW12O404− and lysine moiety. The prepared

eШLysSiW showed good catalytic performance for 4-CP degrada-ion at neutral pH under both the dark and irradiated conditions.he catalyst is stable and can be reused without obvious per-ormance drop. The catalytic activity of FeШLysSiW is relatedo the synergetic effect of ferric iron and SiW12O40

4−, whichakes FeIIILysSiW showing both the Fenton-like catalytic and pho-

ocatalytic activity. The high catalytic activity of FeШLysSiW isssentially ascribed the chemisorption of H2O2 on FeШLysSiW sur-ace by hydrogen bonding, which facilitates both the Fenton-likeatalytic and photocatalytic processes.

cknowledgement

The authors sincerely appreciate National Natural Science Foun-ation of China (51268001) for support of this work.

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