Preparation and catalytic properties of magnetic rectorite-chitosan-Au composites

Article abstract of DOI:10.1016/j.jallcom.2016.08.131

A novel composite was assembled by introducing magnetic Fe₃O₄nanoparticles, chitosan and Au nanoparticles (AuNPs) on rectorite (REC) surfaces. The obtained REC-Fe₃O₄-CTS-Au composite was characterized and used as the catalyst to remove 4-nitrophenol (4-NP) and methyl orange (MO) from water in the presence of NaBH₄. The large surface of REC and the abundant hydroxyl group of chitosan on REC surface could restrain the agglomeration of AuNPs. REC-Fe₃O₄-CTS-Au exhibited the superiority in catalytic efficiency. At the catalyst dosage of 150?mg/L, it took only 15?min for 0.2?mM 4-NP solution to reach complete reduction, and 30?min for 1.0?mM 4-NP solution. This catalyst had the higher catalytic activity for 4-NP than MO reduction. Moreover, the catalyst could be conveniently separated and recycled from the reaction mixtures using an external magnetic field, and reused for 4-NP (or MO) reduction in fourteen cycles with retaining the original 99% (or 95%) conversion efficiency. This work indicates that REC-Fe₃O₄-CTS-Au can be a promising catalyst for the highly efficient degradation of organic dyes.

Full text of DOI:10.1016/j.jallcom.2016.08.131

Accepted Manuscript
Preparation and catalytic properties of magnetic rectorite-chitosan-Au composites
Rufei Zhang, Pengwu Zheng, Xiaofei Ma
PII:
S0925-8388(16)32509-9
10.1016/j.jallcom.2016.08.131
JALCOM 38633
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 12 June 2016
Revised Date: 12 August 2016
Accepted Date: 15 August 2016
Please cite this article as: R. Zhang, P. Zheng, X. Ma, Preparation and catalytic properties of
magnetic rectorite-chitosan-Au composites, Journal of Alloys and Compounds (2016), doi: 10.1016/
j.jallcom.2016.08.131.
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Graphic abstract

ACCEPTED MANUSCRIPT
Preparation and catalytic properties of magnetic
rectorite-chitosan-Au composites
Rufei Zhang a, Pengwu Zheng b, Xiaofei Ma a,*
a Chemistry Department, School of Science, Tianjin University, Tianjin 300072,
China
b School of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang,
Jiangxi 330013, China
Author Information
*Corresponding author; E-mail address: maxiaofei@tju.edu.cn (X.F. Ma);
Tel.: +86 22 27406144; fax: +86 22 27403475.
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Abstract
A novel composite was assembled by introducing magnetic Fe3O4 nanoparticles,
chitosan and Au nanoparticles (AuNPs) on rectorite (REC) surfaces. The obtained
REC-Fe3O4-CTS-Au composite was characterized and used as the catalyst to remove
4-nitrophenol (4-NP) and methyl orange (MO) from water in the presence of NaBH4.
The large surface of REC and the abundant hydroxyl group of chitosan on REC
surface could restrain the agglomeration of AuNPs. REC-Fe3O4-CTS-Au exhibited the
superiority in catalytic efficiency. At the catalyst dosage of 150 mg/L, it took only 15
min for 0.2 mM 4-NP solution to reach complete reduction, and 30 min for 1.0 mM
4-NP solution. This catalyst had the higher catalytic activity for 4-NP than MO
reduction. Moreover, the catalyst could be conveniently separated and recycled from
the reaction mixtures using an external magnetic field, and reused for 4-NP (or MO)
reduction in fourteen cycles with retaining the original 99% (or 95%) conversion
efficiency. This work indicates that REC-Fe3O4-CTS-Au can be a promising catalyst
for the highly efficient degradation of organic dyes.
Keywords: Rectorite; gold; catalyst; 4-nitrophenol; methyl orange
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1. Introduction
Rectorite (REC) is a regularly layered silicate clay mineral comprised of
dioctahedral mica-like layer and dioctahedral smectite-like layer in a ratio of 1:1 [1].
It has attracted a great deal of attention due to its unique properties, such as high
temperature resistance, good dispersion in water, cation exchange ability, adsorption
capacity and adjustable interlayer spacing [2-4]. The large surface area and cation
exchange ability of REC make it a promising candidate for catalyst support in
chemical reaction.
Noble metal nanoparticles (NPs) possess fascinating physicochemical properties
such as optical, electronic, magnetic and biomedical properties, which are extensively
used in catalysis, organic synthesis, storage of hydrogen, biomedical sensors, drug
delivery, and wastewater treatment. Therefore, noble metal NPs have rapidly attracted
great attention in the past decade. [5-9] Among various metal nanoparticles, gold
nanoparticles (AuNPs) have been considered as an attractive and promising catalyst
due to their good recyclability, non-toxic property and excellent catalytic performance
at low temperature. Ismail et al. [10] reported the photochemical deposition of Au
onto mesoporous TiO2 and excellent catalytic activity of the catalyst was confirmed.
Yu et al. [11] synthesized the gold-deposited porous anodic aluminum oxide (AAO)
membranes, which showed high catalytic activity towards reduction of 4-nitrophenol
by NaBH4. Besides, AuNP/poly(methyl methacrylate) beads [12], gold nanoparticle-
graphene hybrids [13], Au-Pd nanoparticles/graphene oxide [14], graphene oxide
aerogels loaded with Au/Pd nanoparticles [15] were also reported to have excellent
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catalytic performance. Among all the AuNP-catalyzed reactions, catalytic reaction of
4-nitrophenol to 4-aminophenol in the presence of sodium borohydride (NaBH4) has
become one of the most widely used model reactions owing to the complete
conversion without by-products and easy measurement of both reactants and products
by UV-Vis spectroscopy [16].
However, significant defects of metal nanoparticles still exist, such as low
catalytic efficiency due to the agglomeration and difficult separation etc. It is a major
and urgent issue to improve the catalytic activity and utilization efficiency of
nano-materials. Hydroxyl groups of chitosan could form interaction with gold ions
and restrain the agglomeration of AuNPs to enhance the dispersion and catalytic
activity of AuNP catalyst [17-19].
With the growing interest in the textile industry, particularly the dye-containing
waste-water and their industrial discharge was reported as one of the major sources of
water pollution worldwide. These dyes may cause permanent injury to the eyes of
humans and animals, and even 4-nitrophenol (4-NP) on inhalation and ingestion may
cause liver and kidney damage [6, 20, 21].
In this work, REC was selected as the support of AuNPs and chitosan
intercalated REC layers with ion exchange method to improve its specific surface area.
Magnetic Fe3O4 NPs were also introduced into the catalyst composites for easy
recovery. On REC surface, hydroxyl groups of chitosan (CTS) could form interaction
with gold ions and restrain the agglomeration of Au nanoparticles to enhance the
dispersion and catalytic activity of AuNP catalyst. The reductions of 4-NP and methyl
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orange (MO) were designed as the model reaction to evaluate the catalytic activity
and efficiency of magnetic Au catalyst composites. The project will set a scientific
foundation for the study of metal nanoparticle supports with large surface area as well
as the recycle and efficiency improvement of metal catalysts.
2. Experimental
2.1. Materials
Sodium rectorite was purchased from Hubei Zhongxiang Rectorite Mine
(Wuhan, China). All other reagents were of analytical grade and commercially
available.
2.2 Preparation of magnetic REC composite
1 g REC was added to a 200 mL solution of 1.17 g FeCl3·6H2O and 0.6 g
FeSO4·7H2O. 20 mL NH3H2O solution (8 M) was added dropwise to produce
iron oxide at 60 ºC under N2. The pH of the final suspension was about 12. The
mixture was held at 70 ºC for 4 h, and then washed with distilled water. The
obtained REC-Fe3O4 composite was dried at 100 ºC for 3 h.
2.3. Fabrication of REC-Fe3O4-CTS composite
1 g REC-Fe3O4 was added to 50 mL distilled water under ultrasonication for
10 min to obtain REC-Fe3O4 suspension. 0.3 g chitosan was added into 30 mL
distilled water and HCl was then added to get a chitosan solution at pH=2. The
REC-Fe3O4 suspension and chitosan solution were mixed and agitated at room
temperature for 30 min. REC-Fe3O4-CTS composite was obtained after washing
with water several times and drying.
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2.4. Preparation of REC-Fe3O4-CTS-Au composite
1 g REC-Fe3O4-CTS composite dispersed in distilled water (50 mL) under
ultrasonication for 10 min. 15 mL 1% HAuCl4 solution was mixed with 12mL
polyvinyl pyrrolidone (PVP) (1 mg/L) as the surfactant, and the mixture was
added to the REC suspension. 8 mL 0.01 mol/L NaOH solution was mixed with
0.24 g NaBH4 and then added dropwise to the above-mentioned mixture at 90ºC
for 5 min to form the composites. The mixture was cooled to room temperature,
washed with distilled water and ethanol, and vacuum-dried at 50ºC for 4 h to get
REC-Fe3O4-CTS-Au composites.
The fabrication process of the catalyst composites was exhibited in Fig. 1.
2.5. Scanning electron microscope (SEM) and Transmission electron microscope
(TEM)
REC, REC-Fe3O4, REC-Fe3O4-CTS and REC-Fe3O4-CTS-Au composites
were stabilized in a sample holder with conducting resin, coated with gold, and
observed with an S-4800 scanning electron microscope.
REC-Fe3O4 and REC-Fe3O4-CTS-Au powders were dispersed in alcohol.
Sample suspensions were dropped into a copper grid, dried in air, and tested with
a JEM-2100F transmission electron microscope.
2.6. X-ray diffraction (XRD)
X-ray diffraction patterns of REC, REC-Fe3O4, REC-Fe3O4-CTS and
REC-Fe3O4-CTS-Au were recorded in reflection mode in the range of 1-80° (2θ)
at room temperature with a D/MAX-2500 operated at a CuKα wavelength of
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1.542Å。
2.7. Fourier transform infrared spectroscopy (FTIR)
FTIR analysis of REC, REC-Fe3O4, REC-Fe3O4-CTS and REC-Fe3O4-CTS-
Au were performed on a BIO-RAD FTS3000 IR spectra scanner. The grinded
sample powders were dispersed in KBr and pressed into transparent sheets for
testing.
2.8. Thermogravimetric analysis (TG)
Thermal properties of REC, REC-Fe3O4, REC-Fe3O4-CTS and REC-Fe3O4-
CTS-Au were measured with a ZTY-ZP thermal analyzer. Samples were heated
from room temperature to 800ºC at a heating rate of 10°/min in nitrogen
atmosphere.
2.9. Specific surface area
The specific surface area of REC, REC-Fe3O4, REC-Fe3O4-CTS and
REC-Fe3O4-CTS-Au were measured with an Autosorb-1 specific surface area
analyzer (Quantachrome Instruments, USA) using the BET method on the base
of N2 adsorption data.
2.10 Magnetic property of REC-Fe3O4-CTS-Au
The magnetic property of REC-Fe3O4-CTS-Au was measured on a vibrating sample
magnetometer (LDJ 9600-1, LDJ Electronics Inc., USA).
2.11 Catalytic activity study of REC-Fe3O4-CTS-Au for 4-NP and MO
1, 3 or 6 mg REC-Fe3O4-CTS-Au was respectively dispersed in 20 mL
4-NP (or MO) aqueous solution containing 0.02 g NaBH4. The dosage of
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REC-Fe3O4-CTS-Au was 50, 150 or 300 mg/L. The mixture was added and
shaken on a rotary shaker at 100 rpm at 30ºC for a certain period of time.
REC-Fe3O4-CTS-Au was magnetically separated from the solution. The
concentration of residual 4-NP (or MO) were measured using UV-Vis
spectrophotometer [22]. To evaluate the catalytic capacity of the REC-Fe3O4-
CTS-Au, the initial concentration of 4-NP changed from 0.2 to 1.0 mmol/L (i.e.
27.8, 55.6, 83.4, 111.3, 139 mg/L), and the initial concentration of MO changed
from 0.03 to 0.09 mmol/L (i.e. 10, 20, 30 mg/L).
To measure the recyclability, 3 mg REC-Fe3O4-CTS-Au and 0.02 g NaBH4
were added to 20 mL aqueous solution, and shaken on a rotary at 100 rpm for 30
min. After REC-Fe3O4-CTS-Au was separated from the solution with the magnet,
the residual 4-NP (or MO) was determined with UV-Vis spectrometry.
Afterwards, REC-Fe3O4-CTS-Au was used for a second catalysis run and the
similar catalytic process was repeated fourteen times. The concentrations of
4-NP and MO were 0.4 mmol/L and 0.06 mmol/L, respectively.
3. Results and discussion
3.1 Characterization of the composites
Fig. 2 (a) revealed that REC was composed of the plates with the size of
about 20 um × 20 um. In Fig. 2 (b), REC-Fe3O4 exhibited Fe3O4 particles
covering the REC sheets. Since Fe3+ and Fe2+ could be adsorbed onto the REC
surface, Fe3O4 particles formed on the transparent plates of REC with the size of
10-40 nm, as shown in Fig. 2 (e). In Fig. 2 (c), some thin REC plates were
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observed, which could be related to the intercalation of acidified chitosan into
REC plates. As shown in Fig. 2 (d) and (f), the scattered gold nanoparticles with
a diameter of below 10 nm were loaded on the surface of REC-Fe3O4. The good
dispersion of gold nanoparticles was also achieved, which was ascribed to the
interaction of polyhydroxy groups of chitosan [23, 24]. As REC-Fe3O4-CTS-Au
could be easily recovered by magnetic field, this composite without obvious
agglomeration of AuNP could achieve better catalytic efficiency and thus was
more preferred.
In Fig. 3 (a), raw REC exhibited strong R(001) and R(002) diffraction
pattern at 2θ=4.10° and 8.11°, respectively. Based on the Bragg diffraction
equation, 2dsinθ=λ, the distance between the REC interlays was calculated to be
about 2.16 and 1.09 nm. After being modified by Fe3O4 and acid-treated chitosan,
the R(001) patterns shifted to 4.00° and 3.62° for REC-Fe3O4 and
REC-Fe3O4-CTS, and the interlayer distance increased to 2.26 and 2.44 nm,
respectively. For the XRD pattern of REC-Fe3O4-CTS, the R(001) reflection of
REC was obviously reduced, and the intensity of the R(002) reflection was
absent. Since REC had a swelling property similar to that of montmorillonite (Mt)
[25], the interlayer Na+ of Mt-like layers in REC could easily exchange with the
+
-NH3 cation parts of chitosan, and chitosan intercalated the REC layers [19]. For
REC-Fe3O4-CTS-Au, both R(001) and R(002) diffraction peaks were absent,
which could be attributed to the partial damage of REC’s orderly structure in the
process of modification (i.e. mostly the intercalation or even exfoliation of REC
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layers) [18].
In the XRD pattern (Fig. 3 (b)), REC-Fe3O4 displayed distinct peaks at 2θ
values of about 30.1, 35.6, 43.3, 57.1 and 62.7. These positions and relative peak
intensities corresponded to the characteristic peaks of Fe3O4 [1]. In the XRD
spectra of REC-Fe3O4-CTS-Au, the peaks at 38.2°, 44.3°, 64.7°, 77.9° were
ascribed to face-centered cubic (fcc) bulk gold of (111), (200), (220), (311),
respectively, which were in accordance with the standard values in the standard
XRD card of gold (JCPDS 04-784) [26]. These peaks confirmed the successful
loading of gold nanoparticles onto REC layers.
Fig. 4 (a) exhibited the FTIR spectra of REC and composites. With regard to REC,
the peaks at 3638 cm-1 was attributed to the hydroxyl stretching band of Si-O-H. The
peak at 1021 cm-1 was related to the in-plane Si-O-Si stretching vibration and the
peaks at 819 and 701 cm-1 were associated with Si-O-Al bending and Al-O-
out-of-plane. The above peaks appeared in the spectra of REC composites, as well.
The broad and intense band of REC-Fe3O4 at 3450 cm−1 was due to stretching
vibrations of hydroxyl groups from iron oxide [27]. In addition, with the introduction
of acid-treated chitosan, the broad stretching vibration of N-H and the stretching
vibration band of -CH2- group at 2935 cm-1 could be easily observed in the spectra of
REC-Fe3O4-CTS and REC-Fe3O4-CTS-Au composites [28].
The thermogravimetric (TG) curves of REC and composites were shown in Fig. 4
(b). The decomposed temperature of chitosan was about 300oC with about 51.7 wt%
mass loss, and REC-Fe3O4-CTS showed a weight loss of about 8 wt% at the
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decomposed temperature of chitosan. The quantity of chitosan was calculated by
matching the percentage weight loss of REC-Fe3O4-CTS to the percentage of weight
loss of chitosan at the decomposed temperature. The content of chitosan in
REC-Fe3O4-CTS was estimated to be about 15.5 wt%. When gold nanoparticles were
loaded on REC-Fe3O4-CTS, the mass loss of REC-Fe3O4-CTS-Au decreased to about
7% at about 300oC. If it was supposed that no chitosan was lost during the process of
loading Au nanoparticles, the decreased mass loss derived from the loading of Au
nonoparticles. The content of Au was calculated to be about 14.3 wt%, by matching
the percentage weight loss (7 wt%) of REC-Fe3O4-CTS-Au to the decreased
percentage (1 wt%) of weight loss of chitosan.
The BET surface area of REC was 10.43 m2/g, while the BET surface area
of REC-Fe3O4 was 77.46 m2/g, much higher than that of raw REC. The increased
surface area of REC-Fe3O4 was attributed to the new-generated nanoscale Fe3O4
particles. When chitosan was introduced, the decline in surface area (53.71 m2/g)
of REC-Fe3O4-CTS was due to the attachment of polymer on the surface of
REC-Fe3O4. However, the nanoscale Au particles only resulted in the slight
improvement of surface area (55.96 m2/g) of REC-Fe3O4-CTS-Au. This could be
ascribed to the partial embedding of AuNPs in chitosan polymers because of the
good interaction between AuNPs and chitosan [23, 24].
Fig. 5 showed the magnetization of REC-Fe3O4-CTS-Au as a function of the
applied magnetic field at 300 K. Magnetization increased with an increase in the
magnetic field. REC-Fe3O4-CTS-Au possessed good magnetic properties with the
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saturation magnetization (5.83 emu/g) and exhibited an extremely small hysteresis
loop and low coercivity (14.0 Oe), as is typically characteristic of superparamagnetic
particles [10].
3.2 Catalytic mechanism
The possible catalytic mechanism for the reduction of 4-NP and MO with
the catalysis of REC-Fe3O4-CTS-Au was presented in Fig. 6. The 4-NP reduction
by NaBH4 in aqueous solution catalyzed by AuNP could be explained by the
Langmuir-Hinshelwood (LH) model [29]. The reaction between the adsorbed
4-NP and the Au-H bonds is the rate-limiting step, compared with the fast
diffusion of the reactants, i.e. borohydride and 4-NP. According to Fig. 6, the
first step involved the hydrolysis of NaBH4 to yield active hydrogen; afterwards,
the active hydrogen was transformed and adsorbed onto the surface of AuNP;
finally, 4-NP reacted with the active H-bond on the surface of AuNP to produce
4-aminopheol. Similarly, MO reacted with the active H-bond to produce N,
N-dimethylaniline and sodium sulfanilate by breaking N=N bond of MO. In
comparison, the numbers of catalyzed NP was two times as much as that of MO
on the surface of one Au particle.
3.3. Catalytic properties of REC-Fe3O4-CTS-Au catalyst
Fig. 7 reveals the effect of contact time on the reduction of 4-NP and MO
over REC-Fe3O4-CTS-Au catalyst in aqueous media at 30oC. Fig. 7 (a) provided
a clear color changes from light yellow (or dark yellow) to colorless for 4-NP (or
MO) with the catalytic times. REC-Fe3O4-CTS-Au could be well dispersed in
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water to catalyze 4-NP (or MO), and this magnetic composite could be easily
recovered with the magnet.
In Fig. 7 (b) for the reduction of 0.4 mmol/L 4-NP, the absorbance of 4-NP
at 400 nm decreased rapidly accompanied by an increase of adsorption peak at
300 nm, which was ascribed to the reducing product, 4-aminophenol. After 25
min, the characteristic peak at 400 nm of 4-NP disappeared, which suggested the
complete catalysis of 4-NP. This indicated that under mild condition the
reduction of 4-NP could achieve a high yield with REC-Fe3O4-CTS-Au as
catalyst. In contrast, the porous 3D Au/graphene monolith could catalyze 0.1
mmol/L 4-NP completely within 25 min [30] and the quaternized chitosan/AgNP
nanocomposite film could catalyze 0.2 mmol/L 4-NP completely within 45 min
[31]. Therefore, REC-Fe3O4-CTS-Au exhibited the superiority in catalytic
efficiency.
In Fig. 7 (c) for the degradation of MO, the absorbance of MO at 465 nm
decreased rapidly accompanied by an increase of adsorption peak at about 248
nm, which related to the combined absorbance from the degraded products (N,
N-dimethylaniline at 243 nm and sodium sulfanilate at 250 nm). After 30 min,
the characteristic peak at 465 nm of MO disappeared due to the complete
catalysis of MO.
To evaluate the catalytic activity of REC-Fe3O4-CTS-Au, the reaction
kinetics of the reduction of 4-NP and MO were studied. The reaction processes
could be described with the dependence of the absorbance (at 400 nm for 4-NP
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and 465 nm for MO) on the contact time. The adopted pseudo-first-order kinetic
model is expressed as follows [31]:












dCt
dt
Ct
A
t
= ln
= ln
= −k t
app
C0
A0
where C0 is the initial concentration of 4-NP (or MO), Ct is the concentration of
4-NP (or MO) at any time (t). A0 is the absorbance of 4-NP (or MO) at t=0, At is
the absorbance at any time (t). The apparent rate constant, kapp, is corresponded
to the catalytic rate, i.e. the higher the apparent rate constant is, the higher the
catalytic rate is. Measured from the linear plot of ln(At/A0) versus time,
according to Fig. 7 (b and c), the rate constant (kapp) value was 0.15 and 0.06
min-1, and the catalytic process fit pseudo-first-order kinetic model well with
correlation coefficient R> 0.99. The results indicated that the catalyst possessed
the higher catalytic activity for 4-NP than MO reduction. On the one hand,
according to catalytic mechanism, MO degradation consumed more the active H
atoms than 4-NP reduction. On the other hand, due to the larger molecular
volume, MO diffused on the catalyst surface more difficultly than 4-NP.
3.4 Effect of the catalyst dosages and dye concentrations on the reduction
As shown in Fig. 8, the effect of REC-Fe3O4-CTS-Au dosage and 4-NP
concentration on the reduction of 4-NP was illustrated in the presence of NaBH4.
At the same catalyst dosage, 4-NP solution with the lower concentration required
less time to reach complete reduction. At the catalyst dosage of 50 mg/L, the
complete reduction took place within 20 min for 0.2 mM 4-NP solution, while
only 70% 4-NP reduction occurred in 30 min for 1.0 mM solution. The catalytic
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efficiency tended to increase while varying the concentration of catalyst from 50
to 300 mg/L. At the catalyst dosage of 150 mg/L, it took only 15 min for 0.2 mM
4-NP solution to reach complete reduction, and 30 min for 1.0 mM 4-NP solution.
However, the catalytic efficiency increased slowly as the catalyst dosage
increased from 150 mg/L to 300 mg/L. Thus, the optimal catalyst dosages are
approximately 150 mg/L for 4-NP reduction.
The influence of catalyst dosage and MO concentration on the degradation
of MO was revealed in Fig. 9. Since the catalytic activity for MO was lower than
4-NP, MO solutions with the lower concentration (0.03 to 0.09 mM) were
catalyzed. At the catalyst dosage of 50 mg/L for 30 min, only 77% MO degraded
for 0.09 mM solution, and about 90% MO degraded for 0.06 mM solution. When
the dosages increase to 150 mg/L, about 90% MO degradation occurred in 30
min. When REC-Fe3O4-CTS-Au dosage was increased to 300 mg/L, above 95%
MO degraded in less 25 min. To catalyze MO solution with higher concentration,
more REC-Fe3O4-CTS-Au catalyst was required to achieve the high catalytic
efficiency (e.g. 95%).
3.5 The recyclability of REC-Fe3O4-CTS-Au
The recyclability test in Fig. 10 (a) showed that complete conversion (99%
of conversion efficiency of 4-NP to 4-aminophenol in 30 min) could be achieved
after fourteen cycles with the catalysis of REC-Fe3O4-CTS-Au. Similarly,
REC-Fe3O4-CTS-Au catalyst could be successfully reused for MO degradation
in fourteen runs with keeping the original 95% conversion efficiency in Fig. 10
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(b). Since no obvious conversion efficiency decreased after 14 runs, more
catalytic cycles were expected to test the recyclability.
As REC had good dispersion in aqueous solution, the recycle process
involving centrifugal separation unavoidably resulted in the loss of the
powder-state catalyst. Loading Fe3O4, chitosan and AuNP on REC not only made
it easier to separate the catalyst for recycle usage, but also enhanced the catalytic
efficiency.
4. Conclusion
The REC-Fe3O4-CTS-Au composite was fabricated by orderly introducing
Fe3O4, chitosan and AuNP catalyst onto the layers of raw REC. As the carrier,
REC layers supported Fe3O4 particles, chitosan and AuNPs. The composite was
easily recovered due to magnetic Fe3O4 particles, and hydroxyl groups of
chitosan could form interaction with metal ions and prevent from the aggregate
of nanoparticles to improve the dispersion and catalytic activity of the composite
catalyst. The REC-Fe3O4-CTS-Au composite was used to catalyze the reduction
of 4-NP and MO in the presence of NaBH4. The REC-Fe3O4-CTS-Au composite
exhibited good catalytic capacity and magnetic recycling properties toward the
reduction of 4-NP and MO, which made it a promising candidate in chemical and
environmental application.
Acknowledgements
This research was supported by the National Nature Science Foundation of
China (51462009).
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Figure captions
Figure 1. The fabrication process of REC-Fe3O4-CTS-Au catalyst.
Figure 2. SEM images of REC (a), REC-Fe3O4 (b), REC-Fe3O4-CTS (c) and
REC-Fe3O4-CTS-Au (d); TEM images for REC-Fe3O4 (e) and REC-Fe3O4-CTS-
Au (f).
Figure 3. X-ray diffraction peaks of REC, REC-Fe3O4, REC-Fe3O4-CTS and
REC-Fe3O4-CTS-Au (1-10o) (a) and (10-80 o) (b).
Figure 4. FTIR spectra (a) and TG curves (b) of REC, REC-Fe3O4, REC-Fe3O4-
CTS and REC-Fe3O4-CTS-Au.
Figure 5. Magnetic behavior of REC-Fe3O4-CTS-Au at 300 K.
Figure 6. Catalytic mechanism of 4-NP and MO reduction.
Figure 7. Catalytic activity of REC-Fe3O4-CTS-Au toward 4-NP and MO.
Time-dependent pictures for the reduction of 4-NP and MO (a); Time-dependent
UV-Vis adsorption spectra for the reduction of 4-NP (b) and MO (c) in aqueous
solution at 30oC. (4-NP and MO concentration: 0.4 mM; catalyst concentration:
150 mg/L)
Figure 8. Effect of the catalyst concentration and 4-NP concentration on the
degradation of 4-NP.
Figure 9. Effect of the catalyst concentration and MO concentration on the
degradation of MO.
Figure 10. The recyclability of REC-Fe3O4-CTS-Au catalysts for the
degradation of 4-NP (a) and MO (b).
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Figure
Figure 1

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Figure 2
(a) REC
(b) REC-Fe3O4
(c) REC-Fe3O4-CTS
(d) REC-Fe3O4-CTS-Au
(e) REC-Fe3O4
(f) REC-Fe3O4-CTS-Au

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Figure 3
(a)
REC-Fe3O4-CTS-Au
REC-Fe3O4-CTS
REC-Fe3O4
REC
0
2
4
6
8
10
2theta (degree)
(b)
REC-Fe3O4-CTS-Au
REC-Fe3O4-CTS
REC-Fe3O4
REC
10
20
30
40
50
60
70
80
2 theta (degree)

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Figure 4
REC-Fe O -CTS-Au
3 4
2931
REC-Fe O -CTS
3 4
1022
2935
REC-Fe O
3 4
3475
1016
3449
REC
701
819
3638
1021
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
100
REC
7%
REC-Fe O
8%
3 4
REC-Fe O -CTS-Au
3 4
REC-Fe O -CTS
3 4
80
60
40
20
51.7%
chitosan
(b)
0
200
400
600
800
Temperature(
)

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Figure 5
6
4
2
0
6
4
-2
-4
-6
2
0
-2
-4
-6
-600
-400
-200
0
200
400
600
-10000
-5000
0
5000
10000
H(Oe)

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Figure 6
H3C
N
H3C
H3C
H3C
Au catalysts
NaBH4,H2O
SO3Na
SO3Na
N
N
N
NH2 + H2N

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Figure 7
4-NP
MO
(a)
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
0min
5min
2.5
2.0
1.5
1.0
0.5
0.0
10min
15min
20min
25min
30min
y= -0.15x-0.8
5
10
15
20
25
30
Time (min)
250
300
350
400
450
500
Wavelength(nm)