Highly efficient reduction of 4-nitrophenolate to 4-aminophenolate by Au/γ-Fe2O3@HAP magnetic composites

Article abstract of DOI:10.1039/c9ra00345b

In this article, the catalyst Au/γ-Fe₂O₃@hydroxyapatite (Au/γ-Fe₂O₃@HAP) consisting of Au nanoparticles supported on the core-shell structure γ-Fe₂O₃@HAP was prepared through a deposition-precipitation method. The catalyst was characterized by transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, N₂ adsorption-desorption and atomic absorption spectrometry. The as-prepared Au/γ-Fe₂O₃@HAP exhibited excellent performance for the reduction of 4-nitrophenolate (4-NP) to 4-aminophenolate (4-AP) in the presence of NaBH₄ at room temperature. Thermodynamic and kinetic data on the reduction of 4-NP to 4-AP catalyzed by the as-prepared catalyst were studied. The as-prepared catalyst could be easily separated by a magnet and recycled 6 times with over 92% conversion of 4-NP to 4-AP. In addition, the as-prepared catalyst showed excellent catalytic performance on other nitrophenolates. The TOF value of this work on the reduction of 4-NP to 4-AP was 241.3 h^-1. Au/γ-Fe₂O₃@HAP might have a promising potential application on the production of 4-AP and its derivatives.

Full text of DOI:10.1039/c9ra00345b

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PAPER
Highly efficient reduction of 4-nitrophenolate to 4-
aminophenolate by Au/g-Fe O @HAP magnetic
2
3
Cite this: RSC Adv., 2019, 9, 10272
composites†
Yide Xia, Ying Liu,* Nannan Shi and Xungao Zhang
*
In this article, the catalyst Au/g-Fe
supported on the core–shell structure g-Fe
method. The catalyst was characterized by transmission electron microscopy, X-ray powder diffraction,
X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, N adsorption–desorption
and atomic absorption spectrometry. The as-prepared Au/g-Fe @HAP exhibited excellent
performance for the reduction of 4-nitrophenolate (4-NP) to 4-aminophenolate (4-AP) in the presence
of NaBH at room temperature. Thermodynamic and kinetic data on the reduction of 4-NP to 4-AP
2
3 2 3
O @hydroxyapatite (Au/g-Fe O @HAP) consisting of Au nanoparticles
2 3
O @HAP was prepared through a deposition–precipitation
2
2
O
3
4
catalyzed by the as-prepared catalyst were studied. The as-prepared catalyst could be easily separated
by a magnet and recycled 6 times with over 92% conversion of 4-NP to 4-AP. In addition, the as-
Received 15th January 2019
Accepted 24th March 2019
prepared catalyst showed excellent catalytic performance on other nitrophenolates. The TOF value of
DOI: 10.1039/c9ra00345b
ꢀ1
this work on the reduction of 4-NP to 4-AP was 241.3 h . Au/g-Fe
2
O
3
@HAP might have a promising
rsc.li/rsc-advances
potential application on the production of 4-AP and its derivatives.
avoid the agglomeration and improve the recovery rate. The
1
. Introduction
19
20
21
populate carriers are metal oxide, carbon materials, silica
22
4
-NP is one of the most common and toxic organic compounds and polymers, which commonly possess large surface area and
in industrial and agricultural wastewater, it is stable and not high stability. It has been conrmed that supported Au catalyst
1
readily degradable in the natural environment. Corresponding exhibits excellent catalytic properties in the reduction of 4-
1
3,23
research indicates that 4-NP can do damage to the ecosystem NP.
and human health. Thus, many approaches to remove 4-NP
2
Magnetic materials with a core–shell structure are oen used
from wastewater have been developed such as photocatalytic as the carrier for fast separation under the effect of magnet.
degradation, microbial degradation, the electro-Fenton Both Fe O and g-Fe O are good magnetic materials, yet they
3 4 2 3
method, microwave-assisted catalytic oxidation and electro- are both sensitive to acid–base environments. A good strategy is
2
4
3
4
5
6
7
coagulation. 4-AP is a very important industrial immediate to encapsulate it into a core–shell structure with a stable
8
25
26
which has been widely used in analgesic and antipyretic drugs,
and as an anticorrosion lubricant and corrosion inhibitor.
material. Common materials of the shells include carbon,
9
10
27
25
A
SiO
2
,
2
TiO (ref. 28) and polymers. HAP is abundant in the
2
9
great deal of works have reported that 4-NP can be transformed nature, easily available and innocuous. It has been widely used
into 4-AP through a catalytic reduction method which proceeds in articial bones, drug deliveries and chemical engi-
30
31
11–13
32
in aqueous solution under mild conditions.
Gold nanoparticles (Au NPs) have been widely used in the between 4 and 12. Many reports show that the Au/HAP has an
34
neering. HAP is the most stable calcium phosphate salt at pH
33
14
15
16
17

eld of energy conversion, sensors, biology and medicine
due to their size-dependent unique properties. However, Au NPs CO oxidation and synthesis of imines and oximes. Never-
are easy to agglomerate due to the high surface energy, which theless, relative researches about Au/g-Fe @HAP catalysts
outstanding performance on the selective oxidation of alcohol,
35
36
O
2 3
18
make the catalytic property of Au NPs decrease. Additionally, and it's catalytic performance for the reduction of 4-NP to 4-AP
Au NPs without being supported or encapsulated by carrier are haven't been reported so far.
difficult to separate from the reaction system for cycle use.
Therefore, Au NPs are usually dispersed onto the solid carrier to as carrier to synthesize Au/g-Fe
In this article, g-Fe
2
O
3
@HAP was synthesized and then used
2 3
O
@HAP through deposition–
precipitation method. The reduction of 4-NP was chosen as the
model reaction to investigate the catalytic property of Au/g-
Fe O @HAP. Inuence of the experiment conditions were dis-
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072,
Hubei, China. E-mail: liuying69@whu.edu.cn; xgzhang66@whu.edu.cn
2
3
cussed, thermodynamic and kinetic researches on the reduc-
tion of 4-NP were studied.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra00345b
1
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the precipitate was centrifuged and washed with deionized
water several times. Precipitate was dried at 363 K overnight and
then calcined at 623 K for 3 h, the color of the precipitate
2
. Experimental section
2.1 Materials
Ferrous chloride tetrahydrate (FeCl
2
$4H
2
O, 99.7%) was changed from dark to brick red, and named as g-Fe
Au/g-Fe @HAP was prepared by deposition–precipitation
Institute (Tian Jin China). o-Nitrophenol (2-NP, 98%) and 4- method. The Au/g-Fe @HAP catalysts with different contents
nitro-m-cresol (98%) were purchased from Aladdin Industrial of Au (1, 2, 3, 4, 6 wt%) were synthesized and labeled as Au/g-
Co., Ltd. (Shanghai China). 3-Nitrophenol (3-NP, 98%) was Fe
@HAP-x (x ¼ 1, 2, 3, 4, 6). Briey, a certain amount of urea
purchased from Saan Chemical Technology Co., Ltd. (Shanghai and 1 g g-Fe @HAP were mixed in the water, stirred at the
China). Iron (III) chloride hexahydrate (FeCl $6H O, 99%), room temperature, then a certain volume of 1 wt% HAuCl
O @HAP.
2 3
purchased from Tian Jin GuangFu Fine Chemical Research
2 3
O
2 3
O
2 3
O
2 3
O
3
2
4
ammonia solution (NH
phosphate ((NH HPO
tetrahydrate (Ca(NO
NaBH , 98%), urea, chloroauric acid tetrahydrate (HAuCl
4H
O) were purchased from Sinopharm Chemical Reagent Co., washed with deionized water three times, dried in the oven at
Ltd. (Shanghai China).
363 K for 10 h, then calcined in the muffle furnace at 573 K for
h.
Element analysis were conducted by atomic adsorption
O, 28%), diammonium hydrogen aqueous solution (molar ratio of urea to Au was 100 : 1) was
3
$H
, 99%), 4-NP (99%), calcium nitrate dropped into the suspension, the total solution volume was
$4H
O, 99%), sodium borohydride kept at 40 mL. Aer 30 min, the temperature was raised to 363 K
and maintained for 3 h. The suspension was centrifuged and
2
4
)
2
4
3
)
2
2
(
4
4
-
$
2
3
2
.2 Characterization
spectrometer, the weight percentages (wt%) of Au of the Au/g-
Fe O @HAP-x were 0.98 wt% (x ¼ 1), 2.02 wt% (x ¼ 2), 3.02 wt%
The morphology and size of sample were characterized by high-
resolution and eld-emission transmission electron micro-
scope (HR-FETEM, JEM2012-FEF). X-ray powder diffraction
2
3
(
x ¼ 3), 4.07 wt% (x ¼ 4), 5.58 wt% (x ¼ 6), respectively.
(XRD) was performed on a diffractometer (X'Pert Pro, PAN-
alytical B.V.) using Cu Ka (l ¼ 0.15406 nm) radiation source at 2.4 Catalytic activity of Au/g-Fe
2 3
O @HAP on the reduction of
a voltage of 40 kV and a current of 40 mA. The corresponding 4-NP
ꢁ
data were collected over a 2q range of 10–80 . The X-ray
1
5 mL of NaBH
aqueous solution, the molar ratio of NaBH
00, stirred at the set temperature. A certain amount of Au/g-
Fe @HAP-x was added to the solution, the reaction began
4
aqueous solution was added into 10 mL of 4-NP
photoelectron spectroscopy (XPS) experiment was carried out
on a X-ray photoelectron spectrometer (Escalab 250Xi, Thermo
Fisher Scientic) with monochromated X-ray, Al Ka (1486.6 eV).
The C1s peak at 284.8 eV of contaminant carbon was taken as
a reference to correct the binding energy scale. Fourier trans-
form infrared spectrum (FT-IR) were done on a FT-IR spec-
trometer (FT-IR 5700, Thermo Fisher Scientic) with KBr discs
method. Specic surface areas and pore size distribution were
4
to 4-NP was kept at
1
2 3
O
and the solution color became shallow gradually. 0.5 mL of the
solution was extracted from the reaction system at the same
interval time, diluted with 2 mL of deionized water, then
transferred into the quartz cuvette and detected by the UV-Vis
2 3
spectrometer immediately. Aer the reaction, Au/g-Fe O @-
computed according to the results of N physisorption at 80 K
2
HAP-x was separated from the system under the attraction of
magnet, washed with deionized water 3 times and used for the
cycle experiment.
using automatic physical adsorption instrument (Tristar II
3020, Micromeritics Instrument Corp) through the BET (Bru-
nauer–Emmet–Teller) method and BJH (Barrett–Joyner–
Halenda) method, respectively. The Au loading amount was
determined by atomic adsorption spectroscopy (AAS) using
atomic adsorption spectrometer (PEAA800, Perkin-Elmer). The
concentration of the substrate was determined by UV-Vis
The extent of the substrate conversions of the above reaction
was calculated according to the eqn (1)
A
0
ꢀ A
t
Conversion ¼
ꢂ 100%
(1)
A
0
spectrophotometer (UV-2550, Shimadzu) according to the where A was the initial absorbance of the substrate solution, A
0
t
Lambert–Beer law.
was the absorbance of the substrate solution at different time
intervals.
2 3
2.3 Synthesis of Au/g-Fe O @HAP
g-Fe O @HAP was prepared according to the ref. 37. Briey,
2
3
3
. Results and discussion
1
.85 mmol of FeCl
dissolved in 30 mL of deionized water, then 10 mL of 28 wt%
NH $H O solution was added slowly under the N atmosphere The morphologies of g-Fe
with strongly stirring at room temperature. The black suspen- were characterized by TEM. Fig. 1(a) shows that g-Fe
sion formed. Aer 30 min, Ca(NO (50 mL, 33.7 mmol) coated with HAP and HAP possesses the well mesoporous
aqueous solution and (NH HPO (50 mL, 20 mmol) aqueous structure. TEM image of Fig. 1(b) shows that Au NPs were
solution, the pH of both which were adjusted to 10 with 28 wt% dispersed well onto the surface of g-Fe O @HAP, the average
2 2 3 2
$4H O and 3.7 mmol of FeCl $6H O were
3.1 Characterization of catalyst
3
2
2
2
O
3
@HAP and Au/g-Fe
2
O
3
@HAP-2
is
2 3
O
3 2
)
4
)
2
4
2
3
ammonia, were dropped into the black suspension simulta- diameter of Au NPs is about 10 nm. As shown in the HRTEM
neously. Aer the drop progress completed, the reaction image of Fig. 1(b), the inter-planar spacing of Au NPs is 0.24 nm,
temperature was adjusted to 363 K and maintained for 3 h, then which corresponds to the Au (111) plane.
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2 3 2 3
Fig. 1 TEM image of (a) g-Fe O @HAP, (b) TEM and HRTEM images Au/g-Fe O @HAP-2.
The curves (a), (b), (c) and (d) shown in Fig. 2 are the FT-IR
absorption spectra of g-Fe , HAP, g-Fe @HAP and Au/g-
Fe @HAP-2, respectively. The peaks of curve (a) at 640 cm
and 575 cm are attributed to the bending vibration of Fe–O in
2
O
3
2 3
O
ꢀ1
2 3
O
ꢀ1
3
7
ꢀ1
g-Fe O . The peak of curve (a) at 2330 cm is the character-
2
3
ꢀ
1
ꢀ1
istic band for CO . The peaks at 3440 cm and 1643 cm are
2
the characteristic peaks of H
adsorbed water. For curve (b), the peaks at 3570 cm and
2
O, which may originate from the
ꢀ
1
ꢀ
ꢀ
1
ꢀ
6
4
1
3
33 cm correspond to the OH vibration of HAP. The peaks at
1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
71 cm , 568 cm , 602 cm , 957 cm , 1039 cm and
ꢀ
ꢀ
1
3ꢀ
093 cm corresponds to the vibration of PO
440 cm and 1643 cm are the characteristic peaks of H O,
4
. The peaks at
1
ꢀ1
2
which are likely to originate from the adsorbed water or crystal
water. It can be seen in Fig. 2, the shape and characteristic peak
of the curve (d) are extremely similar to that of the curve (c),
2 3
which suggests that the structure of g-Fe O @HAP is not
changed aer the Au NPs are introduced.
Fig. 3 shows the XRD patterns of the g-Fe
2 3 2
O , HAP, g-Fe -
2 3 2
Fig. 3 XRD diffraction patterns of (a) g-Fe O , (b) HAP, (c) g-Fe -
O
3
@HAP and Au/g-Fe @HAP-2. The characteristic peaks of
2 3
O
O
3
@HAP and (d) Au/g-Fe
2
O
3
@HAP-2.
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
synthetic g-Fe O at 2q ¼ 30.2 , 35.7 , 43.5 , 54.3 ,57.4 and
2
3
ꢁ
6
3.3 (shown as the curve a in Fig. 3) correspond to the lattice
crystalline planes of (220), (331), (400), (422), (511) and (440),
respectively, which are consistent with the that of standard g-
Fe
2 3
O (JCPDS 25-1402). The synthetic HAP is identied as pure
ꢁ
ꢁ
ꢁ
HAP (JCPDS 09-0432), the peaks at 2q ¼ 26.1 , 31.9 , 33.1 ,
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
34.2 , 40.0 , 46.8 , 49.6 and 53.3 (shown as the curve b in
Fig. 3) correspond to the lattice crystalline planes of (002), (211),
(
Fe
300), (202), (310), (222), (213) and (004), respectively. When g-
ꢁ
2
O
3
is coated with HAP, the peak at 2q ¼ 35.7 appears (shown
as the curve c in Fig. 3) which corresponds to the Fe (331) plane,
it illustrates that the occurrence of the combination of HAP and
g-Fe
@HAP becomes broader, which means that the crystallinity
of HAP in the g-Fe @HAP decreased. Due to the low content
of Au in the Au/g-Fe O @HAP-2, only a weak diffraction peak at
2 3 2
O . Compared to HAP, the characteristic peaks of g-Fe -
O
3
2 3
O
2
3
ꢁ
2
q ¼ 38.2 can be seen (shown as the curve d in Fig. 3), which
Fig. 2 The FT-IR spectra of (a) g-Fe
2 3 2 3
O , (b) HAP, (c) g-Fe O @HAP
corresponds to Au (111) plane (JCPDS 04-0784).
and (d) Au/g-Fe @HAP-2.
2
O
3
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Surface information of the as-prepared Au/g-Fe
2
O
3
@HAP-2 changed. The binding energy of Au 4f7/2 and 4f5/2 were at 83.8 eV
was analyzed by XPS. Fig. 4(a) shows the survey spectrum of and 87.5 eV, respectively, which were consistent with the results
Au/g-Fe O @HAP-2. Fig. 4(b) shows the high-resolution spec- of fresh Au/g-Fe O @HAP-2. It indicated that the Au/g-Fe -
2
3
2
3
2
trum of Fe 2p, the binding energies at 709.4 eV and 722.5 eV are O @HAP-2 posses good stability.
3
2
+
attributable to the Fe 2p3/2 and 2p1/2 in FeO, respectively. FeO
may originate from the partial unoxidized Fe . The binding
energies at 716.5 eV and 728.7 eV correspond to Fe 2p3/2 and IUPAC, the adsorption–desorption isotherms of the two
1/2 satellite peak, respectively. Fe 2p3/2 binding energy is samples belong to type IV isotherms and the hysteresis loops of
Fig. 5(a) shows N
2
adsorption–desorption isotherms g-Fe
2
-
3
O
4
O
3
@HAP and Au/g-Fe
2 3
O @HAP-2, respectively. According to the
2
+
2p
inuenced by ligand eld effects due to the Fe environment (i.e., two samples belonged to H3 types. The types of two samples
tetragonal, octahedral), crystalline disorder at the nanoparticle indicate that the samples contained stack-type pores. The
3+
38
surface, and Russell–Saunders coupling of Fe . g-Fe O
surface areas of g-Fe O @HAP and Au/g-Fe O @HAP-2 are 88.9
2 3 2 3
2
3
3
+
3+
2
ꢀ1
2
ꢀ1
possesses the Fe octahedral and Fe tetrahedral environ- m g and 88.2 m g through BET (Brunauer–Emmett–Teller)
ments. The binding energies at 710.7 eV and 723.9 eV are method, respectively. According to the Fig. 5(b), the average
3
+
indexed to the octahedral Fe 2p3/2 and 2p1/2 in g-Fe
2
O
3
,
2 3 2 3
pore sizes of g-Fe O @HAP and Au/g-Fe O @HAP-2 are
respectively. The binding energies at 712.8 eV and 725.8 eV are computed as 19.1 nm and 18.2 nm through the BJH (Barrett–
3
+
attributable to the tetrahedral Fe 2p3/2 and 2p1/2 in g-Fe
2
O
3
,
Joyner–Halenda) method, respectively.
respectively. The binding energies at 719.8 eV and 732.7 eV
correspond to octahedral Fe 2p3/2 and 2p1/2 satellite peak, O @HAP does not change obviously before and aer the gold
2
The results reveal that the mesoporous structure of g-Fe -
3
+
3
respectively. Fig. 4(c) shows that the binding energies at 83.8 eV loaded.
and 87.5 eV are attributed to Au 4f7/2 and 4f5/2, respectively. It
indicates that the Au specie of Au/g-Fe
2
O
3
@HAP-2 is metallic
39
3.2 Catalytic activity of Au/g-Fe O @HAP on the reduction of
2 3
Au. The Au/g-Fe
2
O
3
@HAP-2 was collected and analyzed with
4
-NP
XPS (see Fig. S1 in the ESI†). The survey spectrum manifests that
the surface elements were not changed aer the catalytic test.
The high resolution spectra of Fe 2p and its tting peak hardly
To evaluate the catalytic activity of Au/g-Fe O @HAP-2, the
reduction of 4-NP to 4-AP was chosen as a model reaction.
2
3
2 3
Fig. 4 (a) XPS survey spectrum, high resolution spectra of (b) Fe 2p and (c) Au 4f of Au/g-Fe O @HAP-2.
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2 2 3 2 3
Fig. 5 (a) N adsorption–desorption isotherms and (b) pore size distribution of g-Fe O @HAP and Au/g-Fe O @HAP-2.
Fig. 6(a) shows the UV-Vis spectra for the catalytic reduction of where C and C
-NP solution catalyzed by Au/g-Fe @HAP-2. It has been re- concentration of 4-NP, respectively. Fig. 6(b) shows the plots of
ported that the wavelength of maximum absorption of 4-NP ln(C/C ) versus reaction time catalyzed by Au/g-Fe @HAP-2
solution is centered at 314 nm and shied to 400 nm in the and g-Fe @HAP. It indicates that g-Fe @HAP barely have
presence of NaBH due to formation of sodium 4-nitro- the catalytic activity on the reduction of 4-NP to 4-AP, thus, Au
0
refers to the reaction as time t and the initial
4
2 3
O
0
2 3
O
2
O
3
2 3
O
4
11
phenolate. The intensity of the peak at 400 nm decreases fast NPs are the active species of the Au/g-Fe O @HAP.
2
3
while the peak at 300 nm appears and its intensity increases
3.2.1 Effect of the amount of catalyst. To investigate the
with the reaction proceeded, the peak at 300 nm belongs to 4-AP inuence of the amount of catalyst, 1 mg, 2 mg, 3 mg, 4 mg and
4
0
that means the 4-NP is transferred into 4-AP. It can be seen 5 mg of Au/g-Fe
that the reaction completes within 12 min. Under the same Fig. 7(a) shows the relationship between C/C
condition, 4-NP could not be effectively reduced to 4-AP with it indicates that the reaction nish time decreases as the
NaBH solely, which is consistent with result of literature re- amount of catalyst increases, which illustrates that the reaction
2
O
3
@HAP-2 were added to the reaction system.
0
and reaction time,
4
41
ported. The initial concentration of NaBH is 100 times than rate increases. Fig. 7(b) shows the linear relationships between
4
that of 4-NP, thus the concentration of NaBH solution can be ln(C/C ) and reaction time when 1 mg, 2 mg and 3 mg of Au/g-
4
0
regarded as a constant and the reaction can be regarded as Fe O @HAP-2 was added, the reaction rate constants are
2
3
42
ꢀ1
ꢀ1
ꢀ1
a pseudo-rst-order reaction. The reaction rate constant (k) 0.206 min , 0.345 min and 0.481 min , respectively. To get
can be calculated with the eqn (2):
more experimental data for monitoring the decline of absor-
bance of 4-NP solution with time, 2 mg is selected as the
(2) optimum amount.
d lnðC=C
dt
0
Þ
¼
ꢀk
2 3 0
Fig. 6 (a) UV-Vis spectra for the catalytic reduction of 4-NP solution catalyzed by Au/g-Fe O @HAP-2, (b) plots of ln(C/C ) versus reaction time
catalyzed by Au/g-Fe
2
O
3
@HAP-2 and g-Fe
2
O
3
@HAP. Reaction conditions: Au/g-Fe
2
O
3
@HAP-2 or g-Fe @HAP (2 mg), 4-NP solution (10 mL,
2 3
O
ꢀ1
ꢀ1
1
mmol L ), NaBH
4
solution (15 mL, 0.067 mol L ) at 293 K.
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Fig. 7 (a) Plots of C/C
0
versus reaction time and (b) plots of ln(C/C
0
) versus reaction time on the reduction of 4-NP at different amounts of Au/g-
ꢀ
1
ꢀ
1
2 3 4
Fe O @HAP-2. Reaction conditions: 4-NP solution (10 mL, 1 mmol L ), NaBH solution (15 mL, 0.067 mol L ) at 293 K.
3
.2.2 Effect of the temperature. The reaction proceeded on the catalysts of Au/g-Fe
different temperatures to investigate the optimum temperature. Fe @HAP-3, Au/g-Fe
Fig. 8(a) shows the relationship between C/C
and reaction time, respectively. The results shows that the reaction rate is too
it indicates that the reaction nish time decreases as the fast when the low loading Au catalysts (Au/g-Fe @HAP-1) was
temperature rise. Fig. 8(b) shows the linear relationships between used, while the reaction rate is slow when the high loading Au
ln(C/C ) and reaction time at different temperature, and the catalysts (>2%) were added. Therefore, Au/g-Fe @HAP-2 is
2
O
3
@HAP-1, Au/g-Fe
2
O
3
@HAP-2, Au/g-
O
O
2
@HAP-4 and Au/g-Fe
O
2 3
@HAP-6,
2
3
3
0
O
2 3
2 3
O
0
ꢀ1
ꢀ1
reaction rate constants are 0.345 min
,
0.446 min
.596 min , 0.674 min and 0.798 min , corresponding to the
temperature of 293 K, 298 K, 303 K, 308 K and 313 K, respectively. relationship between the concentration of 4-NP and the reac-
,
chosen as the optimum catalyst.
ꢀ
1
ꢀ1
ꢀ1
0
3.2.4 Effect of the concentration of 4-NP. To investigate the
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
It illustrates that the reaction rate increases as the increase of tion rate, 2 mmol L , 1.5 mmol L , 1 mmol L , 0.8 mmol L
,
ꢀ
1
ꢀ1
temperature. The optimized temperature was set as 293 K.
.2.3 Effect of the Au loading of catalyst. To investigate the into the reaction system, respectively. Fig. 10(a) shows the
inuence of Au loading of catalyst, the total amount of Au NPs relationship between C/C and reaction time. It indicates that
0.6 mmol L and 0.4 mmol L of 4-NP solution were added
3
0
that the added catalysts contained is kept at 0.04 mg. Fig. 9(a) the reaction rate become faster with the increase of concen-
shows the relationship between C/C0 and reaction time. It tration of 4-NP. Fig. 10(b) shows the good linear relationship
0
indicates that catalyst with low loading of Au has the better between ln(C/C ) and time. The reaction rate constants are
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
catalytic properties on the reduction of 4-NP. Fig. 9(b) shows the 0.468 min
,
0.392 min
,
0.346 min
,
0.317 min
,
ꢀ
1
linear relationships between ln(C/C
reaction rate constants are 0.452 min
0
) and reaction time. The 0.285 min and 0.208 min , corresponding to the 2 mmol
ꢀ
ꢀ1
1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
,
0.345 min
,
L
, 1.5 mmol L , 1 mmol L , 0.8 mmol L , 0.6 mmol L
.114 min , 0.104 min and 0.050 min , corresponding to and 0.4 mmol L of 4-NP solution, respectively.
ꢀ
1
ꢀ1
0
Fig. 8 (a) Plots of C/C
Reaction conditions: 2 mg of Au/g-Fe
0
versus reaction time and (b) plots of ln(C/C
0
) versus reaction time on the reduction of 4-NP at different temperature.
ꢀ1
ꢀ1
2
3
O @HAP-2, 4-NP solution (10 mL, 1 mmol L ), NaBH solution (15 mL, 0.067 mol L ).
4
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Fig. 9 (a) Plots of C/C
0
versus reaction time and (b) plots of ln(C/C
0
) versus reaction time on the reduction of 4-NP over catalysts with different
ꢀ1
4
loading of Au. Reaction conditions: the added catalysts contained 0.04 mg of Au, 4-NP solution (10 mL, 1 mmol L ), NaBH solution (15 mL,
ꢀ
1
0
.067 mol L ) at 293 K.
Fig. 10 (a) Plots of C/C
added 4-NP solution. Reaction conditions: 2 mg of Au/g-Fe
0
versus reaction time and (b) plots of ln(C/C
0
) versus reaction time on the reduction of 4-NP at different concentration of
ꢀ1
2 3
O @HAP-2, 10 mL of 4-NP solution, NaBH solution (15 mL, 0.067 mol L ) at 293 K.
4
According to the above researches, the optimum experiment
conditions are 2 mg of Au/g-Fe O @HAP-2, 10 mL of 1 mmol
2
3
ꢀ
1
ꢀ1
L
4-NP solution, 15 mL of 0.067 mol L NaBH solution at
4
293 K.
3.2.5 Thermodynamic research and diffusion coefficient.
Activation energy is an important parameter in chemical reac-
tions, which can reect the temperature dependency of rate
43
constant for a catalysis reaction. Fig. 11 shows the good linear
relationship between ln k and 1/T. According to the Arrhenius
eqn (3):
E
a
ln k ¼ ꢀ
þ ln A
(3)
RT
where k is the reaction rate constant, E
a
is the activation energy,
R is the universal gas constant, A is the pre-exponential factor.
ꢀ
1
The value of E and A were calculated to be 32.47 kJ mol and
3
a
ꢀ
1
650 s , respectively. It has been reported that the activation
energy of the surface catalyzed reaction is from 8.37 to
Fig. 11 Plots of ln k versus 1/T on the reduction of 4-NP. Reaction
ꢀ1
44
41.84 kJ mol
,
which illustrates that the reduction of 4-NP conditions: 2 mg of Au/g-Fe O @HAP-2, 4-NP solution (10 mL,
2
3
ꢀ1
ꢀ1
occurred on the surface of catalyst in this work.
1 mmol L ), NaBH
4
solution (15 mL, 0.067 mol L ).
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Fig. 12 (a) Photograph of the magnetically separated Au/g-Fe
2
O
3
@HAP-2 from the reaction system after the end of reaction, (b) the relationship
ꢀ1
2 3 4
between conversion and number of runs. Reaction conditions: 2 mg of Au/g-Fe O @HAP-2, 4-NP solution (10 mL, 1 mmol L ), NaBH solution
ꢀ
1
(
15 mL, 0.067 mmol L ).
3
0
.2.6 Recycling and stability of catalyst. Fig. 12(a) shows relationships between ln(C/C ) and reaction time the reduction
the photograph of the magnetically separated Au/g-Fe O @- on these nitrophenolates. The reaction rate constants of 2-NP,
2
3
ꢀ
1
ꢀ1
HAP-2 from the reaction system aer the end of reaction. The 3-NP, 4-nitro-m-cresol and 4-NP are 0.298 min , 0.171 min
,
ꢀ1
ꢀ1
catalyst is easily separated from the reaction system by an 0.278 min and 0.345 min , respectively. It indicates that Au/
external magnet. Fig. 12(b) shows the relationship between g-Fe @HAP-2 possesses favourable universality and practi-
conversion and number of runs. It indicates that the catalyst cability on the reduction of these nitrophenolates.
O
2 3
can be reused 6 times and the conversion maintains over 92%,
which illustrates the catalyst possess excellent stability.
3.2.8 Turnover frequency (TOF) and activity parameter of
the reduction of 4-NP. TOF is usually used for evaluating the
45
3
.2.7 Reduction of other nitrophenolates. 2-NP, 3-NP and catalytic efficiency. TOF is calculated with the following eqn
-nitro-m-cresol were also applied to study the universality and (4):
practicability of the Au/g-Fe O @HAP-2. The reaction experi-
4
2
3
cVM
TOF ¼ mut ꢂ conversion
(4)
mental conditions were 2 mg of Au/g-Fe
2
O
3
@HAP-2, 10 mL of
ꢀ1
ꢀ1
1
mmol L nitrophenolates solution, 15 mL of 0.067 mol L
NaBH solution at 293 K. Fig. 13(a) shows that Au/g-Fe
HAP-2 exhibits excellent catalytic properties on the reduction of
-NP and 4-nitro-m-cresol, but not so well on the reduction of 3-
where c, V, conversion are the initial concentration, volume and
conversion of 4-NP solution, respectively. m, u are the mass and
Au loading of catalyst, respectively. M is the molar mass of gold.
t is the nish time of reaction. The TOF value of this work and
correlative reported work are summarized in Table 1. The TOF
4
2 3
O @-
2
NP. The reduction of these nitrophenolates can also be regarded
as pseudo-rst-order reaction, too. Fig. 13(b) shows that the
Fig. 13 (a) Plots of C/C
-NP. Reaction conditions: 2 mg of Au/g-Fe
at 293 K.
0
versus reaction time and (b) plots of ln(C/C
0
) versus reaction time on the reduction of 2-NP, 3-NP, 4-nitro-m-cresol and
ꢀ1
ꢀ1
solution (15 mL, 0.067 mmol L )
4
O
2 3
@HAP-2, nitrophenolates solution (10 mL, 1 mmol L ), NaBH
4
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a
Table 1 Comparison of parameters of different catalytic systems on the reduction of 4-NP to 4-AP
4
NaBH /4-NP
(equiv.)
ꢀ1
ꢀ1
ꢀ1
Catalysts
Au (mol%)
T (K)
k
c
(L s
g
)
TOF (h
)
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Au/CeO
HPGNPs
K10_A@Au
Au-PDA/RGO
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N
4
6.14
100
0.11
1.62
10
298
298
298
298
298
293
15
333
67
1000
111
100
5.34
4.95
1.12
9.56
1.02
3.56
115.7
88.65
46
55.6
42
47
13
45
48
2
@g-C
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N
4
49
Au/g-Fe
2
O
3
@HAP-2
2
241.3
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a
Au (mol%): the molar ratio of Au to 4-NP.
ꢀ
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