reduction of hexavalent chromium using recyclable pt/pd ...reduction of hexavalent chromium using...

Reduction of Hexavalent Chromium Using Recyclable Pt/PdNanoparticles Immobilized on Procyanidin-Grafted EggshellMembraneMiao Liang,†,§ Rongxin Su,*,†,∥ Wei Qi,†,∥ Yi Zhang,† Renliang Huang,‡ Yanjun Yu,† Libing Wang,†

and Zhimin He†

†State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, and ‡School of EnvironmentalScience and Engineering, Tianjin University, Tianjin 300072, P. R. China§College of Food and Biological Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, P. R. China∥Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China

*S Supporting Information

ABSTRACT: Efficient immobilization of catalytic active metal nanoparticles into porous supporting materials is of importantscientific interest in practice. We report on the fabrication of novel bionanocomposites, comprising a three-dimensional porouseggshell membrane (ESM) bioscaffold decorated with catalytic active metal (Pt, Pd) nanoparticles, to reduce highly toxic Cr(VI).Procyanidin (Pro), a natural plant polyphenol with abundant phenolic hydroxyls, was first covalently grafted on the ESM fibersurface to provide stable binding sites for chelating metal precursors. Highly dispersed Pt and Pd nanoparticles with small sizewere facilely generated and stably immobilized onto the surface of ESM followed by NaBH4 reduction. These metal nanoparticle-incorporating ESM composites were active heterogeneous catalysts for the reduction of toxic Cr(VI) to Cr(III) by employingformic acid as the reducing agent. Notably, it is easy to recover and recycle the catalysts, revealing the good stabilization ofprocyanidin-grafted ESM for nanoparticles.

1. INTRODUCTION

Hexavalent chromium (Cr(VI)) is well-known as an extremelyserious and ubiquitous environmental pollutant produced inwastewaters by several industrial processes such as leathertanning, pigment production, wood preservation, and stainlesssteel manufacturing.1,2 It is considered to be the third mostcommon pollutant at hazardous waste sites and the secondmost abundant heavy metal contaminant.3 Cr(VI) is classifiedas a known human mutagen and carcinogen. Moreover, thehigh environmental mobility of Cr(VI) poses a risk ofgroundwater contamination.4 Generally, the toxicity andwater solubility of chromium are critically dominated by itsoxidation states. Although chromium has many oxidation statesranging from −2 to +6, the predominant oxidation states are +6and +3 in the natural environment.1 Compared with highlywater-soluble and toxic Cr(VI), trivalent chromium (Cr(III)) ismuch less toxic and mobile, tends to form insoluble hydroxides,and can be an essential nutrient for living organisms.5,6 Hence,reductive transformation of Cr(VI) into Cr(III) is a promisingmethod of remediating Cr(VI) contamination, which isfavorable for the environment.7

Various materials and compounds have been employed forthe reduction of Cr(VI) to facilitate environmental remedia-tion, including Fe(0), Fe(II)-bearing minerals, sulfides (S2−),ZnO nanorods, bacterium strains, and several organic matters(such as humic substances, black carbon, and artificial organiccompounds).1,8−12 Additionally, emerging as an important classof environmental catalytic materials, precious metal nano-particles have received considerable attention for theiroutstanding catalytic properties in recent years.13−15 Sadik

and co-workers16 efficiently reduced Cr(VI) to Cr(III) by usingcolloidal palladium nanoparticles as catalyst and formic acid as areducing agent. However, naked nanocatalysts tend toaggregate because of the high surface energy, resulting indecreased catalytic activities. Meanwhile, separation and reuseof the precious metal nanocatalysts is very difficult. Thesedrawbacks may greatly restrict the practical applications ofcolloidal nanocatalysts on environmental remediation. Toaddress these issues, many efforts have focused on thedevelopment of composite catalysts by incorporating metalnanocatalysts on or into solid supports.3,17−19 This approach isvery effective in protecting the nanocatalysts against aggrega-tion and facilitating their recycling.Among many solid supports, porous materials that possess

relatively high surface area are promising for the immobilizationof nanoparticles. For example, Xu et al.3 have demonstratedthat a metal−organic framework could be employed as porousmatrixes to immobilize highly dispersed metal nanoparticles forthe removal of Cr(VI) from wastewater. In addition,mesoporous γ-Al2O3 film and electrospun nanofibrous matshave also been utilized as supports for incorporating Pdnanoparticle (NPs) in the recyclable catalytic reduction ofCr(VI) using formic acid.18,19 Preparation of the porousmatrixes usually involves either sophisticated manipulation orspecial device, which hinders their extensive application.

Received: May 27, 2014Revised: August 9, 2014Accepted: August 13, 2014Published: August 19, 2014

Article

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© 2014 American Chemical Society 13635 dx.doi.org/10.1021/ie5021552 | Ind. Eng. Chem. Res. 2014, 53, 13635−13643

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Therefore, it is highly desirable to develop the novel, applicable,affordable, and environmental friendly porous supports fornanocatalyst immobilization.As one of the nature’s gifts, eggshell membrane (ESM) is a

natural biomembrane with interconnected fibrous structure.The membrane has exhibited great potential as a new biomatrixfor the immobilization of nanoparticles.20 Presently, ESM iscommonly considered as kitchen waste and discarded withoutany treatment, which do not compromise the principle ofsustainable development. In fact, this naturally availablebiomembrane is mainly composed of interwoven collagenprotein fibers. ESM possesses many intrinsic characteristics,such as abundant functional groups for anchoring metalprecursor or convenient chemical modification, high surfacearea for facilitating the loading of nanoparticles, good stabilityin aqueous media, and nontoxicity.21,22 On the basis of thesemerits, a variety of metal and semiconductor nanoparticles hadbeen prepared and deposited onto the ESM fibers.21,23 In ourprevious research, an ESM-supported silver nanocatalyst wasprepared, but it suffered a leaching of silver nanocatalyst and aloss of catalytic activity in the catalytic degradation of anorganic compound.24 This phenomenon may be ascribed to therelatively weak interaction between nanocatalyst and ESM,which is insufficient to provide stability for maintaining thecatalytic properties in the recycling process. Accordingly, it isdesirable to chemically modify the ESM fibers for improvingthe stability of immobilized metal nanocatalysts to achieveefficient and recyclable catalytic reduction of Cr(VI) intoCr(III).Quite recently, we found that grafting of polyphenol onto the

ESM fiber surface could greatly enhance the stability of silvernanoparticles on the ESM matrix,25 prompting us to study thecapacity of surface-modified ESM acting as an efficient supportfor the preparation of stable palladium (Pd) and platinum (Pt)nanocatalysts. In the present study, we synthesize highlydispersed and robustly immobilized metal nanoparticles onprocyanidin-grafted ESM for the efficient and recyclablecatalytic transformation of Cr(VI) into Cr(III) in the presenceof formic acid. Procyanidin (Scheme 1a) is a grape-seed derivedplant polyphenol, and it contains abundant phenolic hydroxylsthat endow it with specific affinity for many metal ions.26,27

Moreover, procyanidin can be grafted onto the surface of ESMfiber by cross-linking of glutaraldehyde28 (Scheme 1b), thussynergistically constructing a stable linkage between the metalprecursor ions and ESM fiber. Therefore, the synthesis ofhighly dispersed and stable Pd and Pt nanoparticles on thesurface of procyanidin-grafted ESM (Pro-ESM) can beexpected following a NaBH4 reduction reaction. Then, varioustechnologies were used to characterize the microstructure and

morphology of the resultant NP-containing ESM compositematerials. We further examined the catalytic activity of thecomposites through the reduction of Cr(VI) into Cr(III) usingformic acid as reducing agent. Meanwhile, the recyclability ofthe Pd and Pt NP-containing ESM was assessed. To the best ofour knowledge, this is the first presentation of the fabrication ofmetal nanoparticles on the surface of ESM for efficient catalyticreduction of Cr(VI) into Cr(III).

2. EXPERIMENTAL SECTION2.1. Materials. Procyanidin dimer was kindly supplied by

JF-Natural Co., Ltd. (Tianjin, China). Fresh eggshells werecollected from a shop in Tianjin University. Potassiumhexachloroplatinate (K2PtCl6, 98%), palladium chloride(PdCl2, 98%), potassium dichromate (K2Cr2O7, 99%),glutaraldehyde (50 wt %), sodium borohydride (NaBH4,98%), and formic acid (88 wt %) were purchased from AladdinReagent Co. (Shanghai, China). Deionized water made fromthe Millipore system was used for all the experiments.

2.2. Preparation of Pro-ESM. The procedure for graftingprocyanidin onto ESM fibers was the same as that in ourprevious work.25 Briefly, 0.5 g of the cleaned and dried ESMpieces (∼5 × 8 mm2) was dispersed into 50.0 mL of deionizedwater and mixed with a certain amount of procyanidin. Themixture was stirred magnetically at 303 K for 2 h. Then, 0.5 mLcross-linking agent of glutaraldehyde at pH 6.5 was added intothe mixture, followed by reacting at 310 K for 6 h undercontinuous magnetic stirring. Then, the product was collected,thoroughly washed with water for removing the unreactedprocyanidin, and dried under vacuum desiccators to get theresultant surface-modified biomatrix Pro-ESM.

2.3. Synthesis of Metal Nanoparticles on Pro-ESM. Thepreparation of stable supported Pt and Pd nanoparticlesincludes the chelating adsorption of metal ions (Pt4+ or Pd2+)onto the phenolic hydroxyls of Pro-ESM, followed by achemical reduction procedure. Typically, the freshly preparedPro-ESM materials were first mixed with 50.0 mL of K2PtCl6 orPdCl2 aqueous solution (4 mM), respectively. After the pH ofthe solution was adjusted to 4.5, the mixture was stirred at 310K for 12 h, allowing the sufficient chelating adsorption of Pt4+

or Pd2+ on Pro-ESM. Then, the metal ion-coordinated Pro-ESM was collected and fully rinsed with water. Theintermediate product was transferred into 10.0 mL of water;then, freshly prepared NaBH4 (400 μL, 1 M) was quicklyintroduced to convert the metal ions into nanoparticles in anice-bath environment. Finally, the resulting metal nanoparticle-immobilized Pro-ESM composites (denoted as MNPs@Pro-ESM; M = Pt, Pd) were again thoroughly washed withdeionized water and then dried in vacuum desiccators.

Scheme 1. Molecular Structure of Procyanidin (a) and Procyanidin Grafted onto ESM Fibers through GlutaraldehydeCrosslinking (b)

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie5021552 | Ind. Eng. Chem. Res. 2014, 53, 13635−1364313636

2.4. Catalytic Experiments. The catalytic efficiency andrecyclability of the as-prepared MNPs@Pro-ESM compositeswere investigated by employing the catalytic reduction ofCr(VI) to Cr(III) in aqueous solution. Typically, 15 mg ofMNPs@Pro-ESM materials was put in a mixture containing 10mL of deionized water and 1 mL of K2Cr2O7 solution (20mM), followed by magnetic stirring at 318 K according to aprevious study.16 Then, 0.8 mL of formic acid was injected tostart the catalytic reaction. During the reaction, aliquots ofmixture were taken out at various times for analyzing thecatalysis efficiency using a UV−vis absorption spectrometer(TU-1810, Persee, China) in the range of 250−700 nm. Aftereach reaction cycle, the MNPs@Pro-ESM composite samplewas washed with water and dried in an oven at 60 °C before thenext catalytic cycle. For comparison, the control experimentwas also conducted in the absence of catalyst or just using Pro-ESM as catalyst under the same experimental conditions.2.5. Characterization Techniques. The morphology of

the MNPs@Pro-ESM composites was evaluated from scanningelectron microscopy (SEM; S-4800, Hitachi Ltd.) images. X-raydiffraction (XRD, D/max 2500, Rigaku) measurement wascarried out using an X-ray diffractometer with a Cu Kα X-raysource. Fourier-transform infrared (FTIR) spectroscopy,thermogravimetric analysis (TGA), and high-resolution trans-mission electron microscopy (HRTEM) analysis were carriedout to characterize the samples using the same procedures asthose in our previous work.25

3. RESULTS AND DISCUSSION

3.1. Preparation of MNPs@Pro-ESM Composites. Inthe present study, an efficient and robust nanoparticle-immobilization biomatrix (Pro-ESM) was first constructed bygrafting of procyanidin onto the ESM fiber surface using cross-linking agent glutaraldehyde. The C6 of the A-ring ofprocyanidin dimer (epicatechin-(4β-8)-epicatechin, Scheme1a) could react with the electrophilic agent glutaraldehydeand form the covalent bond because of its high nucleophilicreaction activity.27 Moreover, the main constituents of ESMfibers are glycoproteins, including collagen and glycosamino-glycans, which are known to possess abundant amino acids.29

Such a structural feature is beneficial for the linking with

glutaraldehyde through the amino groups of the ESM fibers.Therefore, as illustrated in Scheme 1b, a procyanidin-modifiedESM supporting matrix has been fabricated by usingglutaraldehyde as a bridge molecule. In addition, orthophenolichydroxyls of procyanidin can provide abundant and stableanchoring sites for metal precursors. In this sense, procyanidin-grafted ESM could act both as stabilizer and support fornanoparticles.The synthesis procedure of supported metal nanoparticles is

shown in Scheme 2. During the impregnation treatment, metalions (Pt4+ or Pd2+) can be firmly anchored onto the Pro-ESMby the formation of a very stable five-membered chelating ringswith the orthophenolic hydroxyls of procyanidin.26 After theintroduction of NaBH4, the chelated metal ions can be in situreduced to corresponding metal atoms and thus induce thenucleation and growth of PtNPs or PdNPs, respectively. Thesynthesized metal nanoparticles were expected to be stabilizedby procyanidin and located on the surface of Pro-ESM fibers,30

which would facilitate the accessibility of reactant to thecatalytic active surface of supported nanoparticles during thefollowing catalytic reaction.

3.2. Characterization of MNPs@Pro-ESM Composites.Changes in ESM morphology during the surface modificationand nanoparticle synthesis procedure were analyzed by SEM.As shown in Figure 1a, the natural ESM exhibits a macroporousnetwork structure that is composed of interwoven fibers withdiameters between 0.5 and 2 μm. This intricate reticularstructure could provide high specific surface area, which isbeneficial to the immobilization of metal nanoparticles. Also,the presence of a smooth surface for ESM is evident, as shownin Figure 1b. The macroscopic hierarchical structure of ESMwas well maintained (Figure 1c) after the grafting ofprocyanidin. Meanwhile, Figure 1d displays the image of Pro-ESM at a relatively high magnification. A rougher protein fibersurface was found when compared with that of natural ESM,revealing that procyanidin was grafted successfully onto thesurface of ESM fibers via glutaraldehyde cross-linking.Orthophenolic hydroxyl of polyphenols has been proven tobe an excellent bidentate ligand, providing abundant bindingsites for metal ions (Pt4+ or Pd2+) coordination.31 After NaBH4reduction, the color of Pro-ESM changed from pale yellow to

Scheme 2. Schematic Illustration for the Synthetic Methodology of MNPs@Pro-ESM (M = Pt, Pd) Composites



brown, confirming the formation of PtNPs or PdNPs on thePro-ESM matrix. Figure 1e,f and Figure 1g,h display the SEMmorphology of the resultant PtNPs@Pro-ESM and PdNPs@Pro-ESM composites, respectively. The intrinsic interwovenfibrous structure of ESM was effectively maintained. Thisporous structure could allow catalytic reactants to diffuse intothe internal surface of composites and contact with thesupported nanocatalysts effectively. However, the formedPtNPs and PdNPs can be hardly seen from the SEM images,probably because of their small sizes.The EDS elemental mapping analysis (Figure 2) of MNPs@

Pro-ESM composites could confirm that the Pt or Pdnanoparticles were evenly distributed on the Pro-ESM support.In addition, the EDS mapping analysis of the PdNPs@Pro-ESM also revealed the element distribution of C, O, S, and Pthat existed in the proteins of ESM.The size distribution and detailed morphology of the

immobilized metal nanoparticles on the Pro-ESM matrix werefurther examined by TEM analysis. It is difficult to directlyobserve the supported nanoparticles because of the relativelybig thickness of the MNPs@Pro-ESM composites. In thisstudy, the composites were first dispersed in water andsubjected to ultrasonication; then, a drop containing thinfragments of MNPs@Pro-ESM was deposited onto coppergrids for TEM observation. Representative TEM images ofsupported Pt and Pd nanoparticles are shown in panels a and e

of Figure 3, respectively. It can be observed that virtuallyspherical Pt and Pd nanoparticles with few aggregates were alluniformly decorated on the support. The typical HRTEMimages of individual PtNPs (Figure 3b) and PdNPs (Figure 3f)are presented, and a regular lattice fringe of nanoparticles couldbe faintly distinguished, indicating the crystalline nature of theformed metal nanoparticles. Moreover, the size distributionhistograms of PtNPs (Figure 3c) and PdNPs (Figure 3g)suggest that both of the nanoparticles have a narrow sizedistribution. For the synthesized PtNPs, the sizes were almostin the range of 1.8 to 4.2 nm and the mean diameter wasestimated to be 2.83 nm, while for the PdNPs, the particle sizeswere almost 1−3.5 nm with the average diameter of 2.4 nm.These small-sized nanoparticles are expected to possess highcatalytic reactivity because of their great surface area-to-volumeratio.32 According to previous studies,25,33 the polyphenolmolecules (here, procyanidin) facilitate the generation of smallmetal nanoparticles with good distribution on the fibroussupport. The synthesis of metal nanoparticles from chemicalreduction of metal precursor ions includes nucleation andnuclei growth.34 Once the initial metal nuclei were formed,procyanidin could take part in the controlling of metal nucleigrowth and inhibiting the individual particles from coalescingthrough the rigid molecular skeleton of aromatic rings inprocyanidin.25 Moreover, procyanidin-grafted ESM could serve

Figure 1. Scanning electron micrographs of original ESM (a, b), Pro-ESM (c, d), and the as-prepared PtNPs@Pro-ESM (e, f) and PdNPs@Pro-ESM (g, h) composites.

Figure 2. SEM-EDS compositional mapping images of MNPs@Pro-ESM composites (the top-left panel is for PtNPs@Pro-ESM, and theother images are for PdNPs@Pro-ESM), showing the distribution ofrelative elements. Scale bars are 50 μm.



as an efficient stabilizer to prevent the aggregation andmigration of metal nanoparticles, thus giving the small sizeand good dispersion of metal nanoparticles on the support.Hence, an effective biomembrane platform for the synthesisand immobilization of small metal nanoparticles was presentedhere, based on the unique chelating and stabilizing properties ofprocyanidin. The elemental compositions of the resultantMNPs@Pro-ESM composites were also examined usingenergy-dispersive X-ray (EDX) spectrometry. Results exhibitthe peaks for Pt (2.1, 9.4, and 11.1 keV; Figure 3d) and Pd (2.8and 3.0 keV; Figure 3h) elements along with the peaks forelemental S (2.3 keV) and Ca (0.25 and 3.7 keV) from the

ESM,35 supporting the presence of metal nanoparticlesimmobilized on the Pro-ESM biosubstrate.The XRD spectra of the original ESM and MNPs@Pro-ESM

composites are presented in Figure 4a. The naturalbiomembrane exhibited a broad diffraction peak of crystallinedomains at around 20.6°, which can be attributed to theconformations and sequences of amino acids in the ESMprotein fibers.23 The intensity of this peak increased in terms ofthe patterns of MNPs@Pro-ESM composites. Actually, thisphenomenon also occurred in our previous study.25 On thebasis of the fact that the ESM fibers have regularly repeatingamino acid sequences, we consider that both the surfacegrafting of procyanidin using glutaraldehyde as cross-linking

Figure 3. Representative TEM images of MNPs@Pro-ESM composites (a, e); typical HRTEM images of metal nanoparticles showing the latticefringes (b, f); the corresponding size distribution histograms obtained by averaging the sizes of 160 metal nanoparticles (c, g); EDX patterns ofMNPs@Pro-ESM composites (d, h). (Panels a−d are for PtNPs@Pro-ESM; panels e−h are for PdNPs@Pro-ESM).



agent and the in situ formation of nanoparticles could furtherenhance the structural order of ESM fibers, thus leading to theenhancement of XRD peak intensity at around 20.6°.Unexpectedly, the XRD patterns of MNPs@Pro-ESMcomposites did not show the characteristic diffractions formetal nanoparticles, suggesting the formation of very small-sized metal nanoparticles as evidenced by the TEM analysis.This phenomenon is similar to that observed in the

immobilization of silver nanoparticles on ESM24 and theloading of Pt and Pd nanoparticles into porous metal−organicframework.3 Moreover, these results may indicate that most ofthe formed metal nanoparticles were incorporated into theinner fiber surface of ESM. FTIR spectroscopy was employed inan attempt to understand the structural changes during theprocyanidin grafting and the generation of metal nanoparticles.As shown in Figure 4b, characteristic absorption bandscorresponding to the protein amide I (1652 cm−1, COstretching vibration), amide II (1533 cm−1, N−H in planebending/C−N stretching vibration), and amide III (1236cm−1), along with the C−H symmetric and antisymmetricstretching (2925 and 2873 cm−1, respectively) and C−H (1450cm−1, methylene scissor) modes, were observed in each case.This result may imply the protein fiber structure of the ESMsupport was mainly preserved in the grafting and reductionprocesses. Meanwhile, as for the as-prepared Pro-ESM andMNPs@Pro-ESM composites, the appearance of new absorp-tion peaks at 1284 and 1115 cm−1 are caused by the C−O−Cstretching vibration of the benzene ring and the C−O−Hstretching vibration of phenolic hydroxyls in procyanidin,respectively.36 After the formation of metal nanoparticles,however, the stretching vibration of hydroxyls appearingaround 3425 cm−1 (Pro-ESM) shifted to 3386 cm−1, probablybecause of the involvement of the O−H groups in thestabilization of metal nanoparticles.Furthermore, thermal stabilities of the resultant composites

and loading capacity of metal nanoparticles in the Pro-ESMsupport were investigated by TGA under air atmosphere(Figure 4c). All of the samples followed a multistep thermaldecomposition process. The mass losses before 110 °C wereattributed to the water desorption and thermal denaturation ofcollagen. However, the thermal degradation of collagen tookplace in the second stage of decomposition, which starteddecomposing at around 250 °C, and completely decomposed at400 °C. Meanwhile, as can be seen from Figure 4c, MNPs@Pro-ESM composites exhibited a relatively fast decompositionrate compared with that of natural ESM, as also demonstratedpreviously that Pd nanoparticles were immobilized on polymerfibers.19 This phenomenon may be caused by the followingfacts: (i) The incubation of ESM with metal ions and NaBH4treatment during the nanoparticles synthesis process may affectthe structure of collagen to some extent. (ii) The interactionbetween metal nanoparticles and ESM supports may alsofacilitate the pyrolysis of collagen. Moreover, the total Pt andPd content of the composites was roughly determined to be5.91 and 5.66 wt %, respectively, as calculated from thedifference in weight loss.

3.3. Catalytic Reduction of Cr(IV). The suitability of theas-synthesized MNPs@Pro-ESM composites as potentialcatalysts for the transformation of toxic Cr(VI) into Cr(III)has been investigated in aqueous solution using reducing agentof formic acid. Potassium dichromate (K2Cr2O7) was chosen asrepresentative molecule for Cr(VI). It was reported that bothhydrogen donor (formic acid) and chromate were firstadsorbed onto the metal nanoparticle surface where formicacid was decomposed to carbon dioxide and hydrogen. Then,the generated nascent hydrogen reduces Cr(VI) into Cr(III)through hydrogen transfer.The Cr(VI) reduction experiment was performed at 318 K,

and the UV−vis absorption spectra for the reaction in thepresence of MNPs@Pro-ESM (M = Pt, Pd) composites arepresented in panels a and c of Figure 5, respectively. The

Figure 4. XRD patterns of original ESM, Pro-ESM, and the as-prepared MNPs@Pro-ESM composites (a). FTIR spectra (b) oforiginal ESM (spectra A), Pro-ESM (spectra B), PtNPs@Pro-ESM(spectra C), and PdNPs@Pro-ESM (spectra D). TGA curves forsamples of ESM, Pro-ESM, and the resultant MNPs@Pro-ESMcomposites heated from room temperature to 700 °C (10 °Cmin−1)under air atmosphere (c).



successive decrease in the intensity of characteristic absorptionpeak at 350 nm for Cr2O7

2− that is caused by the ligand(oxygen) to metal (Cr(VI)) was found, indicating theconsumption of Cr(VI). Meanwhile, the catalytic performanceof MNPs@Pro-ESM could be visually confirmed by the colorfading from yellow to colorless, indicating the conversion ofCr(VI) (yellow) into Cr(III) (colorless). The presence ofreduction product Cr(III) in the colorless solution was alsoverified by adding NaOH solution to the resulting solution,leading to a green solution because of the generation ofhexahydroxochromate(III).16,37 The reduction reaction wasconsidered complete as the main absorption peak at 350 nmvanished, and this took 15 or 26 min when using PtNPs@Pro-ESM or PdNPs@Pro-ESM as catalyst, respectively. This resultindicated that Pt nanoparticles were more active than Pd for thereduction of Cr(VI), which was consistent with previousreporting of immobilized nanocatalysts.3 Furthermore, controlexperiments to transform Cr(VI) were performed by using Pro-ESM as catalyst with other conditions unchanged. In this case(Figure S1 of the Supporting Information), the catalyticreduction of Cr(VI) did not exhibit efficient spectral change,whereas the slight decrease in peak intensity after 50 min maybe caused by the adsorption capacity of the three-dimensionalporous membrane. Notably, the reduction of Cr(VI) proceededvery slowly when treated with only formic acid and withoutcatalyst.19 These results provided evidence that the trans-formation of Cr(VI) was solely catalyzed by the metalnanoparticles immobilized within the Pro-ESM support.Given that the concentration of formic acid is much greater

than the concentration of Cr(VI) and can be regarded as

constant in this reaction system, pseudo-first-order kinetics withrespect to Cr(VI) could be applied to evaluate the kineticreaction rate of the current reaction. Here, the consumptionrate of Cr(VI) is given by

=−

=rCt

k Cddt

ttapp

where Ct is the concentration of Cr(VI) at time t and kapp is theapparent rate constant of the reaction. The kapp can be obtainedfrom the linear regression of ln (Ct/C0) versus reaction time. Inthis work, the negative logarithm of absorbance (at λ = 350nm) with respect to time (i.e., −ln A350 versus t) was plotted tocalculate the kapp because the absorption intensity of Cr(VI) isproportional to its concentration in the aqueous medium(Figure S2 of the Supporting Information). As shown in theinset of panels a and c of Figure 5, the linear relationshipconfirms the pseudo-first-order kinetics, and the values of kappof the reaction were estimated from the slopes to be 0.196min−1 and 0.133 min−1 for PtNPs@Pro-ESM and PdNPs@Pro-ESM catalysts, respectively. The Pro-ESM supported Pt and Pdnanoparticles in this study possess catalytic activity higher thanthat of metal−organic framework supported metal (Pt and Pd)nanoparticles and PdNPs-doped mesoporous Al2O3films,3,18even if at a lower reaction temperature. Moreover,the turnover frequency (TOF) is an important quality used forassessing catalyst performance. In heterogeneous catalysis, theTOF can be defined as the number of reactant molecules that 1g of catalyst can convert into products per unit time, accordingto previous literature.38 Therefore, in the present study, theinitial TOF was simply calculated to be 1.7 × 1018 and 1.0 ×

Figure 5. Catalytic performance of the MNPs@Pro-ESM composite catalysts. Typical time-dependent UV−vis absorption spectra during thereaction displaying the catalytic transformation of Cr2O7

2− by formic acid (a, c). Inset: the pseudo-first-order plot of −ln A350 versus reaction time.Remaining fraction of Cr2O7

2− with reaction time during the recycling process (b, d). (Panels a and b are for PtNPs@Pro-ESM; panels c and d arefor PdNPs@Pro-ESM).



1018 molecules g−1 s−1 for PtNPs@Pro-ESM and PdNPs@Pro-ESM catalysts, respectively. The exhibited good catalyticproperty of MNPs@Pro-ESM may be attributed to thefollowing aspects: First, the small metal nanocatalysts withuniform distribution usually lead to an increase in the catalyticreactivity because these nanocatalysts with small size couldprovide great surface area-to-volume ratio; hence, more atomson the surface are expected to be available for the catalysis.39

Second, the presence of an interconnected macroporousnetwork in ESM allowed the facile transport of catalyticreactants (formic acid and Cr(VI)) and products to and fromthe active surface of immobilized metal nanocatalysts withoutsuffering high mass-transfer resistance. Therefore, both the sizeeffect of metal nanoparticles and a relatively low diffusionresistance of support were responsible for the good catalyticperformance of MNPs@Pro-ESM composite catalyst.The reusability and recyclability of supported nanocatalysts is

extremely important for successful applications, especially forthe noble metals. In our catalytic system, the macro-dimensional MNPs@Pro-ESM composites can be easilyrecovered from the reaction mixture, washed with water,dried at 60 °C for 20 min, and then subjected to the next cycleof reaction. Panels b and d of Figure 5 show the recyclablereduction of Cr(VI) with PtNPs@Pro-ESM and PdNPs@Pro-ESM as catalyst, respectively, through the plotting of Cr(VI)remaining fraction versus reaction time. As can be seen, thetransformation efficiency of Cr(VI) was still almost 100%within 15 min even in the fifth cycle for PtNPs@Pro-ESM andPdNPs@Pro-ESM could retain 91% productivity within 25 minafter four cycles. Moreover, the kapp values of the reaction byusing MNPs@Pro-ESM catalyst at different cycles werecalculated (Table S1 of the Supporting Information). Therate constant for Cr(VI) reduction increased first and thendecreased when PtNPs@Pro-ESM catalyst was reused. Thisunexpected increase may be caused by the drying procedure at60 °C during catalyst recovery, which may have an activationeffect on the catalyst. As for the PdNPs@Pro-ESM catalyst, therate constant gradually decreased with reused times. Thedecrease in rate constant for Cr(VI) reduction may probably beattributed to the poisoning of the metal nanocrystal surface byadsorption of reactants or products. However, the MNPs@Pro-ESM catalyst exhibited enhanced stability compared with thatof the metal nanoparticles that directly deposited on the naturalESM. These results clearly suggest that MNPs@Pro-ESMcomposite catalysts had good recyclability and stability. Thegood recyclability of composites was attributed to thestabilizing effect of procyanidin toward the generated metalnanoparticles. Accordingly, Pro-ESM could be used as a goodsupporting matrix for the robust immobilization of metalnanoparticles by combining the unique properties ofprocyanidin (strong chelating capacity and stabilization towardmetal ions and the corresponding nanoparticles) and ESM(interconnected fibrous structure, high specific surface area, andphysical robustness). The Pro-ESM supported nanocatalystsoffer an important advantage in terms of low cost, facilesynthesis, easy handling, and reusability and are expected to beuseful in practical applications.

4. CONCLUSIONSIn summary, we have demonstrated the preparation of novelimmobilized metal (Pt and Pd) nanoparticle catalyst byemploying Pro-ESM as an effective supporting matrix andstabilizer. This was achieved by a facile NaBH4 reduction of

metal precursors chelatively adsorbed by procyanidin that wasfirst grafted onto ESM fiber surface. Highly dispersed metalnanoparticles with small size were successfully synthesized andimmobilized into the interwoven fibrous Pro-ESM. Further-more, the resulting MNPs@Pro-ESM composites possessedgood catalytic activity and recyclability for the reduction ofhighly harmful Cr(VI) into Cr(III) with formic acid as reducingagent. By combining the merits of procyanidin and ESM, thiswork provides effective, cost-effective, and environmentalfriendly composite catalysts for the environmental remediationof Cr(VI).

■ ASSOCIATED CONTENT*S Supporting InformationControl experiment for reduction of Cr(VI), calibration curveof Cr(VI), and apparent rate constant of the reaction usingMNPs@Pro-ESM catalyst at different cycles. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Tel: +86 22 27407799. Fax: +86 2227407599.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors gratefully acknowledge financial support from theMinistry of Science and Technology of China (2012YQ090194,2012AA06A303, and 2012BAD29B05), the Natural ScienceFoundation of China (51473115 and 21276192), and theProgram for New Century Excellent Talents in University(NCET-11-0372).

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reduction of hexavalent chromium using recyclable pt/pd ...reduction of hexavalent chromium using...

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