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Aug 19, 2014 - Highly dispersed Pt and Pd nanoparticles with small size were facilely generated and stably immobilized onto the surface of ESM followe...
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Reduction of Hexavalent Chromium Using Recyclable Pt/Pd Nanoparticles Immobilized on Procyanidin-Grafted Eggshell Membrane Miao 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 Environmental Science 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 important scientific interest in practice. We report on the fabrication of novel bionanocomposites, comprising a three-dimensional porous eggshell 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 fiber surface to provide stable binding sites for chelating metal precursors. Highly dispersed Pt and Pd nanoparticles with small size were facilely generated and stably immobilized onto the surface of ESM followed by NaBH4 reduction. These metal nanoparticleincorporating ESM composites were active heterogeneous catalysts for the reduction of toxic Cr(VI) to Cr(III) by employing formic acid as the reducing agent. Notably, it is easy to recover and recycle the catalysts, revealing the good stabilization of procyanidin-grafted ESM for nanoparticles. and co-workers16 efficiently reduced Cr(VI) to Cr(III) by using colloidal palladium nanoparticles as catalyst and formic acid as a reducing agent. However, naked nanocatalysts tend to aggregate because of the high surface energy, resulting in decreased catalytic activities. Meanwhile, separation and reuse of the precious metal nanocatalysts is very difficult. These drawbacks may greatly restrict the practical applications of colloidal nanocatalysts on environmental remediation. To address these issues, many efforts have focused on the development of composite catalysts by incorporating metal nanocatalysts on or into solid supports.3,17−19 This approach is very effective in protecting the nanocatalysts against aggregation and facilitating their recycling. Among many solid supports, porous materials that possess relatively high surface area are promising for the immobilization of nanoparticles. For example, Xu et al.3 have demonstrated that a metal−organic framework could be employed as porous matrixes to immobilize highly dispersed metal nanoparticles for the removal of Cr(VI) from wastewater. In addition, mesoporous γ-Al2O3 film and electrospun nanofibrous mats have also been utilized as supports for incorporating Pd nanoparticle (NPs) in the recyclable catalytic reduction of Cr(VI) using formic acid.18,19 Preparation of the porous matrixes usually involves either sophisticated manipulation or special device, which hinders their extensive application.

1. INTRODUCTION Hexavalent chromium (Cr(VI)) is well-known as an extremely serious and ubiquitous environmental pollutant produced in wastewaters by several industrial processes such as leather tanning, pigment production, wood preservation, and stainless steel manufacturing.1,2 It is considered to be the third most common pollutant at hazardous waste sites and the second most abundant heavy metal contaminant.3 Cr(VI) is classified as a known human mutagen and carcinogen. Moreover, the high environmental mobility of Cr(VI) poses a risk of groundwater contamination.4 Generally, the toxicity and water solubility of chromium are critically dominated by its oxidation states. Although chromium has many oxidation states ranging from −2 to +6, the predominant oxidation states are +6 and +3 in the natural environment.1 Compared with highly water-soluble and toxic Cr(VI), trivalent chromium (Cr(III)) is much 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 promising method of remediating Cr(VI) contamination, which is favorable for the environment.7 Various materials and compounds have been employed for the reduction of Cr(VI) to facilitate environmental remediation, 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 organic compounds).1,8−12 Additionally, emerging as an important class of environmental catalytic materials, precious metal nanoparticles have received considerable attention for their outstanding catalytic properties in recent years.13−15 Sadik © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13635

May 27, 2014 August 9, 2014 August 13, 2014 August 19, 2014 dx.doi.org/10.1021/ie5021552 | Ind. Eng. Chem. Res. 2014, 53, 13635−13643

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

morphology of the resultant NP-containing ESM composite materials. We further examined the catalytic activity of the composites through the reduction of Cr(VI) into Cr(III) using formic acid as reducing agent. Meanwhile, the recyclability of the Pd and Pt NP-containing ESM was assessed. To the best of our knowledge, this is the first presentation of the fabrication of metal nanoparticles on the surface of ESM for efficient catalytic reduction of Cr(VI) into Cr(III).

Therefore, it is highly desirable to develop the novel, applicable, affordable, and environmental friendly porous supports for nanocatalyst 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 biomatrix for the immobilization of nanoparticles.20 Presently, ESM is commonly considered as kitchen waste and discarded without any treatment, which do not compromise the principle of sustainable development. In fact, this naturally available biomembrane is mainly composed of interwoven collagen protein fibers. ESM possesses many intrinsic characteristics, such as abundant functional groups for anchoring metal precursor or convenient chemical modification, high surface area for facilitating the loading of nanoparticles, good stability in aqueous media, and nontoxicity.21,22 On the basis of these merits, a variety of metal and semiconductor nanoparticles had been prepared and deposited onto the ESM fibers.21,23 In our previous research, an ESM-supported silver nanocatalyst was prepared, but it suffered a leaching of silver nanocatalyst and a loss of catalytic activity in the catalytic degradation of an organic compound.24 This phenomenon may be ascribed to the relatively weak interaction between nanocatalyst and ESM, which is insufficient to provide stability for maintaining the catalytic properties in the recycling process. Accordingly, it is desirable to chemically modify the ESM fibers for improving the stability of immobilized metal nanocatalysts to achieve efficient and recyclable catalytic reduction of Cr(VI) into Cr(III). Quite recently, we found that grafting of polyphenol onto the ESM fiber surface could greatly enhance the stability of silver nanoparticles on the ESM matrix,25 prompting us to study the capacity of surface-modified ESM acting as an efficient support for the preparation of stable palladium (Pd) and platinum (Pt) nanocatalysts. In the present study, we synthesize highly dispersed and robustly immobilized metal nanoparticles on procyanidin-grafted ESM for the efficient and recyclable catalytic transformation of Cr(VI) into Cr(III) in the presence of formic acid. Procyanidin (Scheme 1a) is a grape-seed derived plant polyphenol, and it contains abundant phenolic hydroxyls that endow it with specific affinity for many metal ions.26,27 Moreover, procyanidin can be grafted onto the surface of ESM fiber by cross-linking of glutaraldehyde28 (Scheme 1b), thus synergistically constructing a stable linkage between the metal precursor ions and ESM fiber. Therefore, the synthesis of highly dispersed and stable Pd and Pt nanoparticles on the surface of procyanidin-grafted ESM (Pro-ESM) can be expected following a NaBH4 reduction reaction. Then, various technologies were used to characterize the microstructure and

2. EXPERIMENTAL SECTION 2.1. Materials. Procyanidin dimer was kindly supplied by JF-Natural Co., Ltd. (Tianjin, China). Fresh eggshells were collected from a shop in Tianjin University. Potassium hexachloroplatinate (K2 PtCl6, 98%), palladium chloride (PdCl2, 98%), potassium dichromate (K2Cr2O7 , 99%), glutaraldehyde (50 wt %), sodium borohydride (NaBH4, 98%), and formic acid (88 wt %) were purchased from Aladdin Reagent Co. (Shanghai, China). Deionized water made from the Millipore system was used for all the experiments. 2.2. Preparation of Pro-ESM. The procedure for grafting procyanidin onto ESM fibers was the same as that in our previous work.25 Briefly, 0.5 g of the cleaned and dried ESM pieces (∼5 × 8 mm2) was dispersed into 50.0 mL of deionized water and mixed with a certain amount of procyanidin. The mixture was stirred magnetically at 303 K for 2 h. Then, 0.5 mL cross-linking agent of glutaraldehyde at pH 6.5 was added into the mixture, followed by reacting at 310 K for 6 h under continuous magnetic stirring. Then, the product was collected, thoroughly washed with water for removing the unreacted procyanidin, and dried under vacuum desiccators to get the resultant surface-modified biomatrix Pro-ESM. 2.3. Synthesis of Metal Nanoparticles on Pro-ESM. The preparation of stable supported Pt and Pd nanoparticles includes the chelating adsorption of metal ions (Pt4+ or Pd2+) onto the phenolic hydroxyls of Pro-ESM, followed by a chemical reduction procedure. Typically, the freshly prepared Pro-ESM materials were first mixed with 50.0 mL of K2PtCl6 or PdCl2 aqueous solution (4 mM), respectively. After the pH of the solution was adjusted to 4.5, the mixture was stirred at 310 K for 12 h, allowing the sufficient chelating adsorption of Pt4+ or Pd2+ on Pro-ESM. Then, the metal ion-coordinated ProESM was collected and fully rinsed with water. The intermediate product was transferred into 10.0 mL of water; then, freshly prepared NaBH4 (400 μL, 1 M) was quickly introduced to convert the metal ions into nanoparticles in an ice-bath environment. Finally, the resulting metal nanoparticleimmobilized Pro-ESM composites (denoted as MNPs@ProESM; M = Pt, Pd) were again thoroughly washed with deionized water and then dried in vacuum desiccators. 13636

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

glutaraldehyde through the amino groups of the ESM fibers. Therefore, as illustrated in Scheme 1b, a procyanidin-modified ESM supporting matrix has been fabricated by using glutaraldehyde as a bridge molecule. In addition, orthophenolic hydroxyls of procyanidin can provide abundant and stable anchoring sites for metal precursors. In this sense, procyanidingrafted ESM could act both as stabilizer and support for nanoparticles. The synthesis procedure of supported metal nanoparticles is shown in Scheme 2. During the impregnation treatment, metal ions (Pt4+ or Pd2+) can be firmly anchored onto the Pro-ESM by the formation of a very stable five-membered chelating rings with the orthophenolic hydroxyls of procyanidin.26 After the introduction of NaBH4, the chelated metal ions can be in situ reduced to corresponding metal atoms and thus induce the nucleation and growth of PtNPs or PdNPs, respectively. The synthesized metal nanoparticles were expected to be stabilized by procyanidin and located on the surface of Pro-ESM fibers,30 which would facilitate the accessibility of reactant to the catalytic active surface of supported nanoparticles during the following catalytic reaction. 3.2. Characterization of MNPs@Pro-ESM Composites. Changes in ESM morphology during the surface modification and nanoparticle synthesis procedure were analyzed by SEM. As shown in Figure 1a, the natural ESM exhibits a macroporous network structure that is composed of interwoven fibers with diameters between 0.5 and 2 μm. This intricate reticular structure could provide high specific surface area, which is beneficial to the immobilization of metal nanoparticles. Also, the presence of a smooth surface for ESM is evident, as shown in Figure 1b. The macroscopic hierarchical structure of ESM was well maintained (Figure 1c) after the grafting of procyanidin. Meanwhile, Figure 1d displays the image of ProESM at a relatively high magnification. A rougher protein fiber surface was found when compared with that of natural ESM, revealing that procyanidin was grafted successfully onto the surface of ESM fibers via glutaraldehyde cross-linking. Orthophenolic hydroxyl of polyphenols has been proven to be an excellent bidentate ligand, providing abundant binding sites for metal ions (Pt4+ or Pd2+) coordination.31 After NaBH4 reduction, the color of Pro-ESM changed from pale yellow to

2.4. Catalytic Experiments. The catalytic efficiency and recyclability of the as-prepared MNPs@Pro-ESM composites were investigated by employing the catalytic reduction of Cr(VI) to Cr(III) in aqueous solution. Typically, 15 mg of MNPs@Pro-ESM materials was put in a mixture containing 10 mL of deionized water and 1 mL of K2Cr2O7 solution (20 mM), followed by magnetic stirring at 318 K according to a previous study.16 Then, 0.8 mL of formic acid was injected to start the catalytic reaction. During the reaction, aliquots of mixture were taken out at various times for analyzing the catalysis efficiency using a UV−vis absorption spectrometer (TU-1810, Persee, China) in the range of 250−700 nm. After each reaction cycle, the MNPs@Pro-ESM composite sample was washed with water and dried in an oven at 60 °C before the next catalytic cycle. For comparison, the control experiment was also conducted in the absence of catalyst or just using ProESM as catalyst under the same experimental conditions. 2.5. Characterization Techniques. The morphology of the MNPs@Pro-ESM composites was evaluated from scanning electron microscopy (SEM; S-4800, Hitachi Ltd.) images. X-ray diffraction (XRD, D/max 2500, Rigaku) measurement was carried out using an X-ray diffractometer with a Cu Kα X-ray source. Fourier-transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and high-resolution transmission electron microscopy (HRTEM) analysis were carried out to characterize the samples using the same procedures as those in our previous work.25

3. RESULTS AND DISCUSSION 3.1. Preparation of MNPs@Pro-ESM Composites. In the present study, an efficient and robust nanoparticleimmobilization biomatrix (Pro-ESM) was first constructed by grafting of procyanidin onto the ESM fiber surface using crosslinking agent glutaraldehyde. The C6 of the A-ring of procyanidin dimer (epicatechin-(4β-8)-epicatechin, Scheme 1a) could react with the electrophilic agent glutaraldehyde and form the covalent bond because of its high nucleophilic reaction activity.27 Moreover, the main constituents of ESM fibers are glycoproteins, including collagen and glycosaminoglycans, which are known to possess abundant amino acids.29 Such a structural feature is beneficial for the linking with 13637

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Figure 1. Scanning electron micrographs of original ESM (a, b), ProESM (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@ProESM composites (the top-left panel is for PtNPs@Pro-ESM, and the other images are for PdNPs@Pro-ESM), showing the distribution of relative elements. Scale bars are 50 μm.

brown, confirming the formation of PtNPs or PdNPs on the Pro-ESM matrix. Figure 1e,f and Figure 1g,h display the SEM morphology of the resultant PtNPs@Pro-ESM and PdNPs@ Pro-ESM composites, respectively. The intrinsic interwoven fibrous structure of ESM was effectively maintained. This porous structure could allow catalytic reactants to diffuse into the internal surface of composites and contact with the supported nanocatalysts effectively. However, the formed PtNPs 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 Pd nanoparticles were evenly distributed on the Pro-ESM support. In addition, the EDS mapping analysis of the PdNPs@ProESM also revealed the element distribution of C, O, S, and P that existed in the proteins of ESM. The size distribution and detailed morphology of the immobilized metal nanoparticles on the Pro-ESM matrix were further examined by TEM analysis. It is difficult to directly observe the supported nanoparticles because of the relatively big thickness of the MNPs@Pro-ESM composites. In this study, the composites were first dispersed in water and subjected to ultrasonication; then, a drop containing thin fragments of MNPs@Pro-ESM was deposited onto copper grids for TEM observation. Representative TEM images of supported Pt and Pd nanoparticles are shown in panels a and e

of Figure 3, respectively. It can be observed that virtually spherical Pt and Pd nanoparticles with few aggregates were all uniformly decorated on the support. The typical HRTEM images of individual PtNPs (Figure 3b) and PdNPs (Figure 3f) are presented, and a regular lattice fringe of nanoparticles could be faintly distinguished, indicating the crystalline nature of the formed metal nanoparticles. Moreover, the size distribution histograms of PtNPs (Figure 3c) and PdNPs (Figure 3g) suggest that both of the nanoparticles have a narrow size distribution. For the synthesized PtNPs, the sizes were almost in the range of 1.8 to 4.2 nm and the mean diameter was estimated to be 2.83 nm, while for the PdNPs, the particle sizes were almost 1−3.5 nm with the average diameter of 2.4 nm. These small-sized nanoparticles are expected to possess high catalytic reactivity because of their great surface area-to-volume ratio.32 According to previous studies,25,33 the polyphenol molecules (here, procyanidin) facilitate the generation of small metal nanoparticles with good distribution on the fibrous support. The synthesis of metal nanoparticles from chemical reduction of metal precursor ions includes nucleation and nuclei growth.34 Once the initial metal nuclei were formed, procyanidin could take part in the controlling of metal nuclei growth and inhibiting the individual particles from coalescing through the rigid molecular skeleton of aromatic rings in procyanidin.25 Moreover, procyanidin-grafted ESM could serve 13638

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

ESM,35 supporting the presence of metal nanoparticles immobilized on the Pro-ESM biosubstrate. The XRD spectra of the original ESM and MNPs@Pro-ESM composites are presented in Figure 4a. The natural biomembrane exhibited a broad diffraction peak of crystalline domains at around 20.6°, which can be attributed to the conformations and sequences of amino acids in the ESM protein fibers.23 The intensity of this peak increased in terms of the patterns of MNPs@Pro-ESM composites. Actually, this phenomenon also occurred in our previous study.25 On the basis of the fact that the ESM fibers have regularly repeating amino acid sequences, we consider that both the surface grafting of procyanidin using glutaraldehyde as cross-linking

as an efficient stabilizer to prevent the aggregation and migration of metal nanoparticles, thus giving the small size and good dispersion of metal nanoparticles on the support. Hence, an effective biomembrane platform for the synthesis and immobilization of small metal nanoparticles was presented here, based on the unique chelating and stabilizing properties of procyanidin. The elemental compositions of the resultant MNPs@Pro-ESM composites were also examined using energy-dispersive X-ray (EDX) spectrometry. Results exhibit the peaks for Pt (2.1, 9.4, and 11.1 keV; Figure 3d) and Pd (2.8 and 3.0 keV; Figure 3h) elements along with the peaks for elemental S (2.3 keV) and Ca (0.25 and 3.7 keV) from the 13639

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immobilization of silver nanoparticles on ESM24 and the loading of Pt and Pd nanoparticles into porous metal−organic framework.3 Moreover, these results may indicate that most of the formed metal nanoparticles were incorporated into the inner fiber surface of ESM. FTIR spectroscopy was employed in an attempt to understand the structural changes during the procyanidin grafting and the generation of metal nanoparticles. As shown in Figure 4b, characteristic absorption bands corresponding to the protein amide I (1652 cm−1, CO stretching vibration), amide II (1533 cm−1, N−H in plane bending/C−N stretching vibration), and amide III (1236 cm−1), along with the C−H symmetric and antisymmetric stretching (2925 and 2873 cm−1, respectively) and C−H (1450 cm−1, methylene scissor) modes, were observed in each case. This result may imply the protein fiber structure of the ESM support was mainly preserved in the grafting and reduction processes. Meanwhile, as for the as-prepared Pro-ESM and MNPs@Pro-ESM composites, the appearance of new absorption peaks at 1284 and 1115 cm−1 are caused by the C−O−C stretching vibration of the benzene ring and the C−O−H stretching vibration of phenolic hydroxyls in procyanidin, respectively.36 After the formation of metal nanoparticles, however, the stretching vibration of hydroxyls appearing around 3425 cm−1 (Pro-ESM) shifted to 3386 cm−1, probably because of the involvement of the O−H groups in the stabilization of metal nanoparticles. Furthermore, thermal stabilities of the resultant composites and loading capacity of metal nanoparticles in the Pro-ESM support were investigated by TGA under air atmosphere (Figure 4c). All of the samples followed a multistep thermal decomposition process. The mass losses before 110 °C were attributed to the water desorption and thermal denaturation of collagen. However, the thermal degradation of collagen took place in the second stage of decomposition, which started decomposing at around 250 °C, and completely decomposed at 400 °C. Meanwhile, as can be seen from Figure 4c, MNPs@ Pro-ESM composites exhibited a relatively fast decomposition rate compared with that of natural ESM, as also demonstrated previously that Pd nanoparticles were immobilized on polymer fibers.19 This phenomenon may be caused by the following facts: (i) The incubation of ESM with metal ions and NaBH4 treatment during the nanoparticles synthesis process may affect the structure of collagen to some extent. (ii) The interaction between metal nanoparticles and ESM supports may also facilitate the pyrolysis of collagen. Moreover, the total Pt and Pd content of the composites was roughly determined to be 5.91 and 5.66 wt %, respectively, as calculated from the difference in weight loss. 3.3. Catalytic Reduction of Cr(IV). The suitability of the as-synthesized MNPs@Pro-ESM composites as potential catalysts for the transformation of toxic Cr(VI) into Cr(III) has been investigated in aqueous solution using reducing agent of formic acid. Potassium dichromate (K2Cr2O7) was chosen as representative molecule for Cr(VI). It was reported that both hydrogen donor (formic acid) and chromate were first adsorbed onto the metal nanoparticle surface where formic acid 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 the presence of MNPs@Pro-ESM (M = Pt, Pd) composites are presented in panels a and c of Figure 5, respectively. The

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

agent and the in situ formation of nanoparticles could further enhance the structural order of ESM fibers, thus leading to the enhancement of XRD peak intensity at around 20.6°. Unexpectedly, the XRD patterns of MNPs@Pro-ESM composites did not show the characteristic diffractions for metal nanoparticles, suggesting the formation of very smallsized metal nanoparticles as evidenced by the TEM analysis. This phenomenon is similar to that observed in the 13640

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Figure 5. Catalytic performance of the MNPs@Pro-ESM composite catalysts. Typical time-dependent UV−vis absorption spectra during the reaction displaying the catalytic transformation of Cr2O72− by formic acid (a, c). Inset: the pseudo-first-order plot of −ln A350 versus reaction time. Remaining fraction of Cr2O72− with reaction time during the recycling process (b, d). (Panels a and b are for PtNPs@Pro-ESM; panels c and d are for PdNPs@Pro-ESM).

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

successive decrease in the intensity of characteristic absorption peak at 350 nm for Cr2O72− that is caused by the ligand (oxygen) to metal (Cr(VI)) was found, indicating the consumption of Cr(VI). Meanwhile, the catalytic performance of MNPs@Pro-ESM could be visually confirmed by the color fading from yellow to colorless, indicating the conversion of Cr(VI) (yellow) into Cr(III) (colorless). The presence of reduction product Cr(III) in the colorless solution was also verified by adding NaOH solution to the resulting solution, leading to a green solution because of the generation of hexahydroxochromate(III).16,37 The reduction reaction was considered complete as the main absorption peak at 350 nm vanished, and this took 15 or 26 min when using PtNPs@ProESM or PdNPs@Pro-ESM as catalyst, respectively. This result indicated that Pt nanoparticles were more active than Pd for the reduction of Cr(VI), which was consistent with previous reporting of immobilized nanocatalysts.3 Furthermore, control experiments to transform Cr(VI) were performed by using ProESM as catalyst with other conditions unchanged. In this case (Figure S1 of the Supporting Information), the catalytic reduction of Cr(VI) did not exhibit efficient spectral change, whereas the slight decrease in peak intensity after 50 min may be caused by the adsorption capacity of the three-dimensional porous membrane. Notably, the reduction of Cr(VI) proceeded very slowly when treated with only formic acid and without catalyst.19 These results provided evidence that the transformation of Cr(VI) was solely catalyzed by the metal nanoparticles 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

rt =

−dCt = kappCt dt

where Ct is the concentration of Cr(VI) at time t and kapp is the apparent rate constant of the reaction. The kapp can be obtained from the linear regression of ln (Ct/C0) versus reaction time. In this work, the negative logarithm of absorbance (at λ = 350 nm) with respect to time (i.e., −ln A350 versus t) was plotted to calculate the kapp because the absorption intensity of Cr(VI) is proportional to its concentration in the aqueous medium (Figure S2 of the Supporting Information). As shown in the inset of panels a and c of Figure 5, the linear relationship confirms the pseudo-first-order kinetics, and the values of kapp of the reaction were estimated from the slopes to be 0.196 min−1 and 0.133 min−1 for PtNPs@Pro-ESM and PdNPs@ProESM catalysts, respectively. The Pro-ESM supported Pt and Pd nanoparticles in this study possess catalytic activity higher than that of metal−organic framework supported metal (Pt and Pd) nanoparticles and PdNPs-doped mesoporous Al 2 O 3 films,3,18even if at a lower reaction temperature. Moreover, the turnover frequency (TOF) is an important quality used for assessing catalyst performance. In heterogeneous catalysis, the TOF can be defined as the number of reactant molecules that 1 g of catalyst can convert into products per unit time, according to previous literature.38 Therefore, in the present study, the initial TOF was simply calculated to be 1.7 × 1018 and 1.0 × 13641

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1018 molecules g−1 s−1 for PtNPs@Pro-ESM and PdNPs@ProESM catalysts, respectively. The exhibited good catalytic property of MNPs@Pro-ESM may be attributed to the following aspects: First, the small metal nanocatalysts with uniform distribution usually lead to an increase in the catalytic reactivity because these nanocatalysts with small size could provide great surface area-to-volume ratio; hence, more atoms on the surface are expected to be available for the catalysis.39 Second, the presence of an interconnected macroporous network in ESM allowed the facile transport of catalytic reactants (formic acid and Cr(VI)) and products to and from the active surface of immobilized metal nanocatalysts without suffering high mass-transfer resistance. Therefore, both the size effect of metal nanoparticles and a relatively low diffusion resistance of support were responsible for the good catalytic performance of MNPs@Pro-ESM composite catalyst. The reusability and recyclability of supported nanocatalysts is extremely important for successful applications, especially for the noble metals. In our catalytic system, the macrodimensional MNPs@Pro-ESM composites can be easily recovered from the reaction mixture, washed with water, dried at 60 °C for 20 min, and then subjected to the next cycle of reaction. Panels b and d of Figure 5 show the recyclable reduction of Cr(VI) with PtNPs@Pro-ESM and PdNPs@ProESM as catalyst, respectively, through the plotting of Cr(VI) remaining fraction versus reaction time. As can be seen, the transformation efficiency of Cr(VI) was still almost 100% within 15 min even in the fifth cycle for PtNPs@Pro-ESM and PdNPs@Pro-ESM could retain 91% productivity within 25 min after four cycles. Moreover, the kapp values of the reaction by using MNPs@Pro-ESM catalyst at different cycles were calculated (Table S1 of the Supporting Information). The rate constant for Cr(VI) reduction increased first and then decreased when PtNPs@Pro-ESM catalyst was reused. This unexpected increase may be caused by the drying procedure at 60 °C during catalyst recovery, which may have an activation effect on the catalyst. As for the PdNPs@Pro-ESM catalyst, the rate constant gradually decreased with reused times. The decrease in rate constant for Cr(VI) reduction may probably be attributed to the poisoning of the metal nanocrystal surface by adsorption of reactants or products. However, the MNPs@ProESM catalyst exhibited enhanced stability compared with that of the metal nanoparticles that directly deposited on the natural ESM. These results clearly suggest that MNPs@Pro-ESM composite catalysts had good recyclability and stability. The good recyclability of composites was attributed to the stabilizing effect of procyanidin toward the generated metal nanoparticles. Accordingly, Pro-ESM could be used as a good supporting matrix for the robust immobilization of metal nanoparticles by combining the unique properties of procyanidin (strong chelating capacity and stabilization toward metal ions and the corresponding nanoparticles) and ESM (interconnected fibrous structure, high specific surface area, and physical robustness). The Pro-ESM supported nanocatalysts offer an important advantage in terms of low cost, facile synthesis, easy handling, and reusability and are expected to be useful in practical applications.

metal precursors chelatively adsorbed by procyanidin that was first grafted onto ESM fiber surface. Highly dispersed metal nanoparticles with small size were successfully synthesized and immobilized into the interwoven fibrous Pro-ESM. Furthermore, the resulting MNPs@Pro-ESM composites possessed good catalytic activity and recyclability for the reduction of highly harmful Cr(VI) into Cr(III) with formic acid as reducing agent. By combining the merits of procyanidin and ESM, this work provides effective, cost-effective, and environmental friendly composite catalysts for the environmental remediation of Cr(VI).

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ASSOCIATED CONTENT

* Supporting Information S

Control experiment for reduction of Cr(VI), calibration curve of Cr(VI), and apparent rate constant of the reaction using MNPs@Pro-ESM catalyst at different cycles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 22 27407799. Fax: +86 22 27407599. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Ministry of Science and Technology of China (2012YQ090194, 2012AA06A303, and 2012BAD29B05), the Natural Science Foundation of China (51473115 and 21276192), and the Program for New Century Excellent Talents in University (NCET-11-0372).

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REFERENCES

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4. CONCLUSIONS In summary, we have demonstrated the preparation of novel immobilized metal (Pt and Pd) nanoparticle catalyst by employing Pro-ESM as an effective supporting matrix and stabilizer. This was achieved by a facile NaBH4 reduction of 13642

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

Industrial & Engineering Chemistry Research

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