Article
pubs.acs.org/IECR
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:
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May 27, 2014
August 9, 2014
August 13, 2014
August 19, 2014
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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.
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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
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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
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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
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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
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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 ×
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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).
■
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.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Tel: +86 22 27407799. Fax: +86 22
27407599.
Notes
The authors declare no competing financial interest.
■
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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