Palladium nanoparticles supported on core-shell and yolk-shell Fe3O4@nitrogen doped carbon cubes as a highly efficient, magnetically separable catalyst for the reduction of nitroarenes and the oxidation of alcohols

Article abstract of DOI:10.1016/j.jcat.2018.05.003

The preparation of palladium nanoparticles (Pd NPs) supported on Fe₃O₄@nitrogen doped carbon (N-C) core-shell (C-S) and yolk–shell (Y-S) nanostructures is reported. The Fe₃O₄@N-C@Pd C-S nanostructures were synth

Full text of DOI:10.1016/j.jcat.2018.05.003

Journal of Catalysis 364 (2018) 69–79
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Palladium nanoparticles supported on core-shell and yolk-shell
Fe O @nitrogen doped carbon cubes as a highly efficient, magnetically
3
4
separable catalyst for the reduction of nitroarenes and the oxidation of
alcohols
⇑
Siyavash Kazemi Movahed, Noushin Farajinia Lehi, Minoo Dabiri
Faculty of Chemistry and Petroleum Sciences, Shahid Beheshti University, Tehran 1983969411, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation of palladium nanoparticles (Pd NPs) supported on Fe O @nitrogen doped carbon (N-C)
3
4
Received 21 January 2018
Revised 30 April 2018
Accepted 2 May 2018
core-shell (C-S) and yolk–shell (Y-S) nanostructures is reported. The Fe
3
O
4
@N-C@Pd C-S nanostructures
were synthesized by two different methods. The first method included two steps: (i) the annealing treat-
ment of the Fe @polydopamine (PDA) under a gas mixture of Ar/H flow and (ii) the decorating Pd NPs
on the surface of annealed structures by sodium borohydride as a reducing reagent. The second method,
2
O
3
2
the in-situ reduction of Fe
flow as a reducing reagent led to Fe
tures were synthesized by using a HCl solution as an effective etching reagent to the partially etch the
Fe . Four types of Pd-based catalysts were tested in the selective reduction of p-nitrophenol (p-NP)
2
O
3
and Pd2 in the structure of Fe
+
2 3
O @PDA@Pd by annealing treatment under
2+
Keywords:
Carbon
Reduction
Oxidation
H
2
3 4
O @N-C@Pd C-S nanostructure. Additionally, the Y-S nanostruc-
3 4
O
Magnetic separation
Nanocubes
Heterogeneous catalysis
to p-aminophenol (p-AP). Different catalytic activities were obtained through these catalysts.
Furthermore, the reduction of nitroarenes and the oxidation of benzylic alcohols were also studies.
Ó 2018 Elsevier Inc. All rights reserved.
1
. Introduction
the involvement of iron in the redox processes causes a significant
3 4
decrease in catalytic activity [16]. Therefore, the Fe O have been
Tailoring the morphology of inorganic nanocrystal’s have long
preferentially coated with catalytically inert materials such as
polymer, carbon or silica to support the transition metal nanopar-
ticles [17].
been an attractive field of nanoscience and technology [1]. In par-
ticular, magnetic NPs with unique size- and shape-dependent mag-
netic properties inspired an intensive exploration of their potential
applications in the past decade [2,3]. Numerous composites have
been extensively investigated for their magnetic nanostructures
Dopamine (DA) is a biomolecule that contains catechol and
amine functional groups, can self-polymerize at alkaline pH values
under the oxygen atmosphere. The conformal and continuous
polydopamine (PDA) shell can be spontaneously fixed onto various
substrates and nanostructures such as hematite, ferrite, etc. due to
the high binding affinity of catechol derivative anchor groups (phe-
nolic hydroxyl) to iron oxide just through immersing the particles
into the dopamine solution at room temperature [18]. It is also an
excellent carbon source that can yield thin N-doped carbon coat-
ings with similar structures and electrical conductivities compared
to that of N-doped multilayered graphene. Metal salts could be
readily adsorbed onto PDA surfaces or absorbed into PDA nanos-
tructures, leading to metal-containing N-doped carbon nanostruc-
tures by carbonization [19].
including pure metals (Fe, Co, and Ni), metal oxides (Fe
-Fe ), MFe (M = Cu, Ni, Mn, and Mg), and metal alloys (FePt
3 4
and CoPt) [4,5]. Among these magnetic NPs, ferrite (Fe O )
3 4
O and
c
2
O
3
2 4
O
nanoparticles have been widely used experimentally for numerous
applications such as in the separation of biomolecules [6], drug and
gene targeting [7], tissue engineering [8], magnetic resonance
imaging [9], magnetic biosensors [10], biocatalysts [11], wastewa-
ter treatment [12], supports in heterogeneous catalysts [13], and as
mediators of heat for cancer therapy [14]. The naked NPs tend to
aggregate to form larger particles because of Van der Waals forces,
resulting in a significant deterioration of their original activity [15].
When metal (0) NPs are supported on the bare surface of ferrite,
Palladium catalysts have been developed, and widespread
applications in organic transformations, including oxidation [20],
alkylation [21], hydrogenation [22], hydroformylation [23], car-
bonylation [24], and cross-coupling reactions [25]. As a result of
⇑
Corresponding author.
E-mail address: m-dabiri@sbu.ac.ir (M. Dabiri).
https://doi.org/10.1016/j.jcat.2018.05.003
021-9517/Ó 2018 Elsevier Inc. All rights reserved.
0

7
0
S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
the toxicity and high price of Pd, in recent years, interest has grown
in the organization and use of recoverable heterogeneous Pd cata-
lysts. For this purpose, different solid materials such as modified
silicas [26], zeolites [27] molecular sieves [28], polymers [29],
mesoporous materials [30], ionic liquids [31], activated carbon
In the first method, the Fe
Y-S (A) structures were obtained from reducing H
NaBH over Fe @N-C C-S and Fe @N-C Y-S structures, respec-
tively. For the second method, the Fe @N-C@Pd C-S (B) was
obtained from the in-situ reduction of the Fe
and Pd2+ in the
as a reducing reagent.
@N-C@Pd Y-S (B) was synthesized via the
core of Fe @N-C@Pd C-S (B) nanos-
3
O
4
@N-C@Pd C-S (A) and Fe
3
O
4
@N-C@Pd
2
PdCl with
4
4
3
O
4
3 4
O
3 4
O
2 3
O
2
+
[
32], and natural supports [33] have been used as supports for
structure of Fe
2 3 2
O @PDA@Pd by using H
the heterogenization of Pd catalysts.
Furthermore, the Fe O
partial etching of the Fe
tructure using HCl.
3 4
Core-shell (C-S) and yolk–shell (Y-S) structured materials with
tailored physical and chemical properties have shown great poten-
tial for a variety of applications including catalysis, drug delivery,
energy storage and conversion [34–39]. Based on this introduction
and also as part of our ongoing studies on the synthesis of carbon-
based nanocomposite catalysts [40], herein, we report the success-
3
O
4
3 4
O
2. Experimental
ful synthesis of unique Fe
proposed synthetic strategy for the preparation of Fe
C-S and Y-S is illustrated in scheme 1. The uniform Fe
were coated with PDA to obtained Fe @PDA C-S cubes followed
by transforming into Fe @N-C C-S cubes after an annealing treat-
ment under a gas mixture of Ar/H flow. The Fe @N-C Y-S was
synthesized by the partial etching of the Fe core in Fe @N-
C C-S structure by using HCl. The Fe @N-C@Pd C-S and Fe
N-C@Pd Y-S structures were prepared using two different methods.
3
O
4
@N-C@Pd C-S and Y-S cubes. The
@N-C@Pd
cubes
2.1. Instrumentation
3 4
O
2
O
3
Diffraction data were collected on a STOE STADI P with scintil-
lation detector, secondary monochromator and Cu-Ka1 radiation (k
= 1.5406 Å). Field emission scanning electron microscopy (FESEM)
images were characterized using an electron microscope ZEISS
SIGMA VP-500. Transmission electron microscopy (TEM) images
were characterized using a transmission microscope Philips CM-
30 electron microscope with an accelerating voltage of 150 kV.
2 3
O
3 4
O
2
3 4
O
3
O
4
3 4
O
3
O
4
3 4
O @
3 4 3 4 3 4 3 4
Scheme 1. Schematic illustration of the formation procedure of Fe O @N-C@Pd C-S (A), Fe O @N-C@Pd Y-S (A), Fe O @N-C@Pd C-S (B), and Fe O @N-C@Pd Y-S (B)
nanostructures.

S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
71
X-ray photoelectron spectroscopy (XPS) analysis was performed
2.2.6. Synthesis of Fe
3
O
4
@N-C@Pd C-S (A) and Fe
@N-C C-S or Fe
3
O
4
@N-C@Pd Y-S (A)
using a Thermo Scientific, ESCALAB 250Xi with Mg X-ray resource.
The concentrations of iron and palladium were estimated using
inductively coupled plasma optical emission spectrometer (ICP-
OES) Varian Vista PRO Radial. The nitrogen adsorption-desorption
The 100 mg of Fe
3
O
4
3
O
4
@N-C Y-S was dispersed
PdCl solution
L of HCl and bring
volume to 5 mL) was added dropwise over 10 min. After stirring for
1 h, the NaBH solution (17.70 mg dissolved in 5 mL of deionized
water) was added dropwise over 10 min, and stirred for another
2.5 h. The Fe @N-C@Pd was collected by external magnetic field.
in 100 mL of deionized water by sonication. The H
(prepared by dissolving 8.3 mg of PdCl in 6.8
2
4
2
l
isotherms were collected using a Micromeritics MicroActive for
4
1
TriStar II Plus 2.03 surface area analyzer with N
2
at 77 K. H NMR
spectra were recorded on a Bruker DRX-300 AVANCE spectrometer
at 300.13 MHz. UV/Vis spectra were recorded employing an Ana-
lytik Jena Specord S600 Diode Array spectrometer. Gas chromatog-
raphy was performed on a Trace GC ultra from the Thermo
3 4
O
The structure was washed with water (200 mL) and ethanol (200
mL) and dried at 70 °C overnight.
Ò
Company equipped with FID detector and Rtx -1 capillary column.
2
.2.7. Synthesis of Fe
The 100 mg of Fe
deionized water by sonication. The H
dissolving 8.3 mg of PdCl in 6.8 L of HCl and bring volume to
mL) was added dropwise over 10 min followed by vigorously
3
O
4
@N-C@Pd C-S (B)
@PDA C-S was dispersed in 100 mL of
PdCl solution (prepared by
The magnetic properties of the prepared nanocomposites were
measured using a homemade LBKFB model vibrating sample mag-
netometer (Meghnatis Daghigh Kavir Company, Iran) at r.t.
2 3
O
2
4
2
l
5
2
2
.2. Material synthesis
stirring for 3 h at r.t. The resulting material was washed three
times with deionized water (200 mL) and ethanol (200 mL) and
dried at 70 °C overnight. The prepared Fe O @PDA@Pd was
2 3
heated in a furnace at 360 °C for 5 h under a continuous flow Ar–
.2.1. Material
2+
Commercially available iron(iii) chloride hexahydrate (99%),
sodium hydroxide (99%), tris(hydroxymethyl)aminomethane
hydrochloride (99%), hydrochloric acid (37%), palladium(ii) chlo-
ride (99%), potassium carbonate (99%) and sodium borohydride
À1
H
2
(9:1 as v/v%) with a heating rate of 3 °C min to transform into
3 4
Fe O @N-C@Pd C-S (B).
(
(
(
98%) were purchased from Merck Millipore. 1-Phenylethanol
98%), 4-chlorobenzyl alcohol (99%), 4-methylbenzyl alcohol
98%), 4-nitrobenzyl alcohol (99%), benzyl alcohol (99%), diphenyl-
2.2.8. Synthesis of Fe
The annealed Fe
3 4
(2 M) solution for 1 h. The Fe O
3
O
O
3 4
4
@N-C@Pd Y-S (B)
@N-C@Pd C-S (B) was stirred in 100 mL HCl
@N-C Y-S was collected by external
methanol (99%), 1-indanol (98%), dopamine hydrochloride (99%),
3
4
-nitrophenol (98%), 4-nitrophenol (99%), 2-nitrophenol (98%),
-nitroanisole 97%, 4-nitrotoluene (99%) and 1-chloro-4-
magnetic field and washed with water and ethanol and dried at 70
°C overnight.
nitrobenzene (99%) were purchased from Sigma-Aldrich.
-(Dimethylamino)benzyl alcohol (97%) and nitrobenzene (99%)
were purchased from TCI. All the chemical reagents were used as
received without further purification. Deionized water was used
throughout the experiment.
4
2
.2.9. General procedure for the reduction of p-nitrophenol (p-NP)
The 30 mL of p-NP (0.12 mM) was mixed with 30 mL of a freshly
4
prepared aqueous NaBH solution (0.17 M) to form a yellow solu-
tion. The catalyst (1 mol% of Pd) was added to the resulting solu-
tion under open air conditions, and the reaction conversion was
monitored by UV–Vis spectroscopy.
2
.2.2. Synthesis of Fe
The Fe cubes were synthesized by a precipitation method. In
a typical synthesis, 50 mL of 5.4 M NaOH solution was added to 50
mL of 2.0 M FeCl solution within 5 min under continuous stirring
at 75 °C. The resultant Fe(OH) gel was continuously stirred at the
2 3
O cubes
2 3
O
3
2.2.10. General procedure for the reduction of nitroarenes
3
30 mL of 0.12 mM nitroarene (H
0.17 M NaBH (H O:EtOH (1:1)) were mixed in a 100 mL round-
bottom flask. Then, 2 mg of Fe @N-C@Pd Y-S (B) (1 mol% of
Pd) was added to the resulting solution. The mixture was stirred
vigorously at r.t under open air conditions. The progress of the
reaction was monitored by thin-layer chromatography (TLC). After
the completion of the reaction, the catalyst was separated from the
reaction mixture by external magnetic field and washed with etha-
nol and ethyl acetate. The reaction mixture was concentrated, and
2
O:EtOH (1:1)) and 30 mL of
same temperature for 5 min and was then aged in a preheated
oven at 100 °C for 4 days (in a sealed 250 mL round-bottom flask).
The red product was collected and washed three times with deion-
ized water (300 mL) and ethanol (300 mL) and dried at 70 °C over-
night [41].
4
2
3 4
O
2 3
2.2.3. Synthesis of Fe O @PDA C-S
8
0 mg of Fe cubes was dispersed into 100 mL of Tris-buffer
2 3
O
2
the residue was purified by thin-layer chromatography on SiO (n-
Hexane:Ethyl acetate 8:2 as v:v%). The solvent was removed under
vacuum to yield pure product.
solution (10 mM) under sonication for 30 min. Afterward, 40 mg
of dopamine was slowly added to the mixture solution under son-
ication for 50 min which was then vigorously stirred for 4 h with
magnetic stirring. The resultant product was collected via centrifu-
gation and washed with deionized water (200 mL) and ethanol
2
.2.11. General procedure for the oxidation of benzyl alcohols
A mixture of K CO (1 mmol) and 2 mg of Fe @N-C@Pd Y-S
B) (1 mol% of Pd) in PhCH (5 mL) was prepared in a two-
necked rounded bottom flask. A solution of the benzyl alcohol (1
mmol) in PhCH (5 mL) was injected into the solution, and the
(
200 mL) for three times and dried at 70 °C overnight.
2
3
3 4
O
(
3
2
.2.4. Synthesis of Fe @N-C C-S
3 4
O
Fe
2
O
3
@PDA C-S was heated in a furnace at 360 °C for 5 h under
3
À1
a continuous flow of Ar–H (9:1) with a heating rate of 3 °C min
3 4
to transform into the Fe O @N-C C-S.
2
resulting mixture was stirred at 90 °C under air. The progress of
the reaction was monitored by thin-layer chromatography (TLC).
After the completion of the reaction, the catalyst was separated
from the reaction mixture by external magnetic field and washed
with ethanol and ethyl acetate. Then the reaction mixture was con-
centrated, and then the residue was purified by using thin layer
2
.2.5. Synthesis of Fe
The annealed Fe
solution for 2 h. The Fe
3
O
O
3 4
4
@N-C Y-S
@N-C C-S was stirred in 100 mL HCl (2 M)
@N-C Y-S was collected by external mag-
3 4
O
netic field and washed with water (200 mL) and ethanol (200 mL)
chromatography over SiO
2
(n-Hexane:Ethyl acetate 9:1 as v:v%).
and dried at 70 °C overnight.
The solvent was removed under vacuum to yield pure product.

7
2
S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
3
. Result and discussion
Brunauer–Emmett–Teller (BET) surface area, average pore vol-
umes, and average pore sizes are summarized in Table 1. After dec-
orating Pd NPs on the surface of Fe @N-C structures, the surface
3.1. Characterization
3 4
O
area increases, which the Pd NPs played as the spacer to between
structures and thereby preventing their agglomeration [43,44].
The Y-S structures showed a higher BET surface area than the C-S
structures, which indicates the formation of hollow void in Y-S
The morphologies of the samples were characterized by trans-
mission electron microscope (TEM) (Fig. 1). Fe O @N-C C-S parti-
3 4
cles presented a C-S structure with the N-C shell thickness of
about 30 nm (Fig. 1a and b). After the etching the structure with
2
structures. As shown in Fig. 3, the N isotherms of all structures
HCl for 2 h, hollow voids were created in Fe
Fig. 1c and d). The surface of Fe @N-C@Pd C-S (A) and Fe
C@Pd Y-S (A) were decorated with Pd NPs with the size of about
0–15 nm (Fig. 1e–f and g–h). However, aggregated Pd NPs were
observed in these samples. The size of the Pd NPs was increased
to about 25–30 nm without aggregation in the Fe @N-C@Pd C-
S (B) and Fe @N-C@Pd Y-S (B) structures (Fig. 1i–l). These results
indicated that the particle size of Pd NPs supported by the NaBH
reduction method smaller than those supported by the H reduc-
3
O
4
@N-C Y-S structure
displayed a type-IV curve manifesting both micro/mesoporosity,
which possibly originates from the gasification of various raw
materials during the pyrolysis [45,46].
(
3
O
4
3 4
O
@N-
1
The XRD patterns of Fe
@N-C@Pd C-S (A), Fe
S (B), and Fe @N-C@Pd Y-S (B) structures were shown in
Fig. S1. The XRD pattern of Fe showed diffraction peaks
2
O
3
, Fe
3
O
4
@N-C C-S, Fe
3
O
4
@N-C Y-S,
Fe
3
O
4
3
O
4 3 4
@N-C@Pd Y-S (A), Fe O @N-C@Pd C-
3
O
4
3 4
O
O
4
2 3
O
3
positions at 2h = 24.14°, 33.15°, 35.61°, 49.48°, 54.10°, 62.45°,
and 63.99° can be attributed to the (0 1 2), (1 0 4), (1 1 0), (0 2 4),
4
2
tion method. These results are in agreement with a previous report
in reduction metal NPs [42].
Additionally, the morphologies of the samples were character-
ized by the field emission scanning electron microscope (FESEM)
(1 1 6), (2 1 4), and (3 0 0) planes of Fe
absence of the Fe peaks in the XRD patterns after thermal
annealing treatment demonstrated the transformation from
Fe to Fe . All of the annealed structures showed diffraction
peaks of Fe without visible Fe residue. The diffraction peaks
for the Pd nanoparticles are not detected. Additionally, the peaks at
2h = 44° and 64° correspond to the stainless-steel sample holder of
the powder diffractometer.
2 3
O , respectively. The
2 3
O
2
O
3
3 4
O
(
Fig. 2). The cubic structure was maintained after the annealing
treatment in all of the samples. Moreover, the aggregated Pd NPs
were detected in the prepared samples by NaBH as the reducing
reagent (Fig. 2c and d), in accordance with TEM observation.
sorption isotherms were used to investigate the surface
areas and pore structures of the as-prepared structures. The
3
O
4
2 3
O
4
N
2
The chemical composition of the yolk-shell structures was char-
acterized by X-ray photoelectron spectroscopy (XPS). As shown in
Fig. 1. TEM images of the (a and b) Fe
3 4 3 4 3 4 3 4 3 4
O @N-C C-S, (c and d) Fe O @N-C Y-S, (e and f) Fe O @N-C@Pd C-S (A), (g and h) Fe O @N-C@Pd Y-S (A), (i and j) Fe O @N-C@Pd C-S (B),
(
k and l) Fe @N-C Y-S (etching time: 1 h) (B) structures.
3 4
O

S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
73
Fig. 2. FESEM images of the (a) Fe
3
O
4
@N-C C-S, (b) Fe
3
O
4
@N-C Y-S, (c) Fe
3
O
4
@N-C@Pd C-S (A), (d) Fe
3
O
4
@N-C@Pd Y-S (A), (e) Fe
3
O
4
@N-C@Pd C-S (B), and (f) Fe
3
O
4
@N-C@Pd
Y-S (B) structures.
Table 1
2
N adsorption–desorption analysis results.
Structure
Surface area (m /g)
Average pore volume (cm /g) 0.01
Average pore size (nm) 6.3
Fe
3
O
4
@N-C C-S Fe
3
O
4
@N-C Y-S Fe
3
O
4
@N-C@Pd C-S (A) Fe
3
O
4
@N-C@Pd Y-S (A) Fe
3
O
4
@N-C@Pd C-S (B) Fe
3
O
4
@N-C@Pd Y-S (B)
2
7.96
28.57
0.09
12.8
16.82
0.05
11.2
27.41
0.09
12.5
12.75
0.03
9.5
36.66
0.13
14.4
3
2
Fig. 3. N adsorption–desorption isotherms.
Fig. S2a and e, the peaks corresponding to C 1s, Fe 2p, N 1s, O 1s, Pd
d were observed in the XPS full spectra. The high-resolution XPS
spectra of the N 1s core level, it can be deconvoluted into three
component peaks at the binding energy of about 398.3, 399.7,
and 400.5 eV, corresponding to pyridinic N, pyrrolic N, and quater-
nary N, respectively [47] (Fig. S2b and f). These results indicated
that PDA transformed into nitrogen-doped carbon by carboniza-
tion process. Moreover, the N atomic content percent on the N-C
3

7
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S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
Fig. 4. The VSM curve of structures.
t 0
Fig. 5. Plot of ln(C /C ) vs. reaction time for the reduction of p-NP.
Table 2
The constant rate k the reduction of p-NP.
Structure
Fe
3
O
4
@N-C@Pd C-S (A)
Fe
3
O
4
@N-C@Pd Y-S (A)
Fe
3
O
4
@N-C@Pd C-S (B)
Fe
3 4
O @N-C@Pd Y-S (B)
À1
Rate constant k (min
)
0.0328
0.1618
0.2667
0.9626

S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
75
Table 3
a
Optimization of the reaction conditions.
Yieldb (%)
TOF (h )
À1
Entry
Solvent
Catalyst (mol of Pd%)
Time (min)
1
2
3
4
5
H
EtOH
EtOH:H
EtOH:H
EtOH:H
EtOH:H
2
O
1
1
0.5
1
2
45
270
45
45
45
72
79
47
81
84
Trace
96
17.6
125.3
108
56
2
O (1:1)
2
O (1:1)
2
O (1:1)
2
O (1:1)
c
6
–
1440
–
a
b
c
3
4
0 mL of 0.12 mM nitrobenzene (solvent) and 30 mL of 0.17 M NaBH (solvent), r.t, under air.
Isolated yields.
Without catalyst.
Table 4
3 4
The scope of Fe O
@N-C@Pd Y-S (B) catalyzed reduction of nitroarenes.a
Yieldb (%)
81
TOF (h
108
À1
)
Entry
1
Nitroarene
Aniline
Time (min)
45
2
3
7
3
98
98
840.5
1960
4
5
6
110
120
45
80
84
85
43.7
42
113.3
a
3
0 mL of 0.12 mM nitroarene (H
2 4 2 3 4
O:EtOH (1:1)), 30 mL of 0.17 M NaBH (H O:EtOH (1:1)), Fe O @N-C@Pd Y-S (B) (1 mol% of Pd), r.t., under air.
b
Isolated yields.
shell of Fe
tures was calculated to be 5.06 and 5.88 at%, respectively, accord-
ing to XPS. The core level region of Pd 3d for the Fe @N-C@Pd Y-S
A) and Fe @N-C@Pd Y-S (B) structures included two compo-
nents. The first one is constituted by two doublets, situated at
3
O
4
@N-C@Pd Y-S (A) and Fe
3
O
4
@N-C@Pd Y-S (B) struc-
showed in Fig. S2c and g, the two peaks at 725.2 and 711.5 eV,
are assignable to Fe 2p1/2 and Fe 2p3/2 for Fe , respectively, that
indicated the transformation from Fe to Fe [52].
The magnetic properties of the Fe @N-C C-S, Fe
@N-C@Pd C-S (A), Fe @N-C@Pd Y-S (A), Fe @N-C@Pd C-S
(B), and Fe @N-C@Pd Y-S (B) structures were investigated using
a vibrating sample magnetometer (VSM) at room temperature
(Fig. 4). The magnetic saturation (Ms) values of the Fe @N-C
C-S, Fe @N-C Y-S, Fe @N-C@Pd C-S (A), Fe @N-C@Pd Y-S
(A), Fe @N-C@Pd C-S (B), and Fe
3 4
O
O
3 4
3
O
4
2
O
3
(
3
O
4
3
O
4
3 4
O @N-C Y-S,
Fe
3
O
4
3
O
4
3 4
O
0
3
35.3 and 340.6 eV, attributable to Pd that arises due to the reduc-
3 4
O
2
+
tion of Pd by reducing reagents (NaBH
4 2
(Fig. S2c) and H
0
(
Fig. S2g)) and proved that Pd mainly exists as Pd . N-C layer
3 4
O
increases the binding energy of Pd atom (in contrast with 3d5/2
of metallic Pd 335.0 eV [48]). This result shows a charge transfer
between Pd and the carbon support, and agreement with a previ-
3
O
4
3
O
4
3 4
O
3
O
4
3
O
4
@N-C@Pd Y-S (B) structures
À1
was 67.8, 62.8, 59.2, 44.7, 69.9, and 55.9 emu g , respectively.
d+
ous report on the presence of an additional Pd contribution in
After depositing Pd NPs on the surface of Fe
3 4
O @N-C C-S and
the Pd/C catalyst [49]. The second component, located at 337.3
and 342.4 eV, is attributable to the Pd2+, which may be ascribed
to the oxidation of surface atoms exposed to air. The synergistic
action of the Pd and PdO species may play an important role in
highly active catalytic species in the oxidation of alcohols [50]
and reduction of nitroarenes [51]. The core level regions of Fe 2p
Fe @N-C Y-S structures, the saturation magnetization decreased.
3 4
O
This can be explained by considering the probable reduction in the
dipolar–dipolar interactions between the magnetic nanoparticles
[53]. Additionally, the lower Ms values of Y-S structures were
due to the partial etching Fe
of the iron in these structures. Also, the Fe
3
O
4
core with HCl and the low loading
@N-C@Pd C-S (B)
3 4
O

7
6
S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
Fig. 6. (a) Reusability of the Fe
3
O
4
@N-C@Pd Y-S (B) nanostructure as the catalyst in ten consecutive cycles for the reduction of p-NP (b) TEM image of Fe
3
O
4
@N-C@Pd Y-S (B)
nanostructure after ten cycles.
Table 5
a
Optimization of the reaction conditions.
Yieldb (%)
Conversionb (%)
Carbonyl selectivityb (%)
Carboxyl selectivityb (%)
TOF (h
À1
)
Entry
Solvent
Temp. (°C)
Base
1
2
3
4
5
6
PhCH
DMF
3
90
90
Reflux
90
90
90
90
90
80
100
90
90
90
90
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
3
92
51
38
26
40
88
45
94
79
92
25
32
59
–
96
55
41
38
44
94
51
99
82
96
27
35
63
–
99
99
99
72
99
96
99
99
99
99
99
99
99
–
–
–
–
27
–
3
–
–
–
–
–
–
–
–
9.2
5.1
3.8
2.6
4.0
8.8
9
6.3
7.9
9.2
2.5
3.2
5.9
–
CH
3
CN
2
H O
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
3
3
3
3
3
3
3
3
3
3
DBU
K
K
K
K
K
K
K
K
K
3
2
2
2
2
2
2
2
2
PO
4
c
7
8
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
d
9
1
1
1
1
1
0
1
2
3
4
e
f
g
h
a
b
c
3 4
Benzyl alcohol (2 mmol), base (1 eq.), solvent (5 mL), n-dodecane as an internal standard (0.1 mL) and Fe O @N-C@Pd Y-S (B) (1 mol% of Pd), 10h, air.
Determined by GC analysis.
Fe
Fe
Fe
Fe
Fe
3
O
3
O
3
O
3
O
3
O
4
@N-C@Pd Y-S (B) (0.5 mol% of Pd).
4
@N-C@Pd Y-S (B) (1.5 mol% of Pd).
4
@N-C@Pd C-S (A) (1 mol% of Pd).
4
@N-C@Pd Y-S (A) (1 mol% of Pd).
4
@N-C@Pd C-S (B) (1 mol% of Pd).
d
e
f
g
h
Without catalyst.
showed higher Ms values than the Fe
could be related to the low carbon content in Fe
B) structures [54].
3
O
4
@N-C@Pd C-S (A) that
addition of the catalyst, the intensity of the characteristic absorp-
tion peak of p-nitrophenolate ion decreased, and the feature
absorption of p-AP at around 300 nm also appears (Fig. S6b–e).
3 4
O
@N-C@Pd C-S
(
t 0
As shown in Fig. 5, a linear relationship between ln(C /C ) vs. reac-
tion time was obtained in the reduction catalyzed, which indicates
that the reduction follows pseudo-first order reaction kinetics. The
constant rates k for the reduction of p-NP catalyzed by using 1.0
3
.2. The catalytic activity of Fe
3 4 3 4
O @N-C@Pd C-S and Fe O @N-C@Pd
Y-S structures on the reduction of nitroarenes
mol% Pd loading of Fe
3 4 3 4
O @N-C@Pd C-S (A), Fe O @N-C@Pd Y-S
4
The reduction of p-NP by NaBH to investigate the catalytic per-
formance. As shown in Fig. S6a, the p-NP solution exhibited a
strong UV absorption band at 317 nm which shifts to 400 nm after
(
A), Fe @N-C@Pd C-S (B), and Fe
3
O
4
3
O
4
@N-C@Pd Y-S (B) nanostruc-
À1
tures are 0.0328, 0.1618, 0.2667, and 0.9626 min , respectively
(Table 2). The higher rate constants in Y-S structures than
4
the addition of freshly prepared NaBH solution, due to the forma-
the C-S structures could be due to the nanorattle structure
tion of p-nitrophenolate anion under alkaline conditions. On the

S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
77
Table 6
3 4
Aerobic oxidation of alcohols catalyzed by Fe O
@N-C@Pd Y-S (B) nanostructure.a
Yieldb (%)
92
Conversionc (%)
96
Carbonyl selectivityc (%)
99
TOF (h
9.2
À1
)
Entry
1
Alcohol
Aldehyde
2
3
87
74
93
79
99
8.7
7.4
>97
4
90
61
89
94
66
95
>99
>99
>99
9.0
6.1
4.5
5
6d
7d
8d
94
92
98
96
97
4.7
4.6
>99
a
b
c
Reaction conditions: alcohol (2 mmol), K
Isolated yield.
Based on GC analysis.
2
CO
3
(2 mmol), PhCH
3
(5mL), 10 h, 90 °C, air.
d
2
0 h.
characteristics that indicated the large free reaction voids inside
the Y-S structures [40]. Also, the results indicated that the catalytic
activity of catalysts prepared by the method B higher than those
prepared by the method A. On the other hand, the activity of the
Pd catalysts generally increased with an increase in the Pd particle
size. A similar particle size effect has been reported for the hydro-
genation of 1-nitrohexane over Pd/mesoporous silica catalysts [55]
as well as for the hydrogenation of 2,4-dinitrotoluene (2,4-DNT)
over Pd/C catalysts [56]. It could be suggested that a different
adsorption geometry of the substrate on particles of different size
could influence the rate and the distribution products [56].
Another explanation for this size effect, it may be related to the
increase of the hydrogen adsorption with the Pd particle sizes [55].
To optimize the reaction conditions, a series of experiments
with different parameters were performed for the reduction of
nitrobenzene. The effect of the solvent was investigated, and
and the yield is 79% in 270 min (Table 3, entry 2). It can be seen
that the mixed solvent EtOH:H O (v:v = 1:1) was chosen as the
best solvent in the reduction of nitrobenzene (Table 3, entry 4).
The reason is that the mixed solvent of EtOH:H O (v:v = 1:1) can
make both the nitrobenzene and the NaBH completely dissolved.
The effect of the amount of catalyst was also investigated under
the optimum reaction conditions (Table 3, entries 3–5). When
the amount of catalyst was increased from 0.5 to 1 and 2 mol%,
the yield increased from 47% to 81% and 84%, respectively. The con-
centration of the catalyst of 1 mol% was sufficient for the reaction
to occur (Table 3, entry 4). Probably, the agglomeration of catalyst
particles occurred for high catalyst amount, which led to decreased
2
2
4
4
the number active catalyst sites (Table 3, entry 5). NaBH was not
able to reduce nitrobenzene even after one day without catalyst
(Table 3, entry 6).
3 4
Next, the scope of the well-defined catalyst Fe O @N-C@Pd Y-S
results showed in Table 3. It can be seen that, using H
vent, the yield is just 72% in 45 min (Table 3, entry 1), partially due
to the insolubility of the nitrobenzene in H O. When using EtOH as
cannot be completely dissolved
2
O as the sol-
(B) was examined in the reduction of various nitroarenes possess-
ing both electron-withdrawing and donating substituents. 2- and
3-nitrophenols gave good yields of anilines in a very short amount
of time (Table 4, entries 2–3). Nitroarenes possessing
2
the reaction solvent, the NaBH
4
Table 7
The reusability of the Fe
3 4
O
@N-C@Pd Y-S (B) nanostructure in aerobic oxidation of benzyl alcohol.a
Reaction cycle
1st
92
2nd
92
3rd
90
4th
89
5th
86
6th
84
7th
83
8th
80
9th
80
10th
76
Yieldb (%)
a
Benzyl alcohol (7 mmol), K
Yield determined by GC analysis.
2
CO
3
3 3 4
(7 mmol), PhCH (20 mL), n-dodecane (0.2 mL) and Fe O @N-C@Pd Y-S (B) nanostructure (1 mol% of Pd), 10 h, 90 °C, air.
b

7
8
S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
electron-donating groups such as p-OMe and p-Me and electron-
withdrawing group such as p-Cl also afforded good yields of the
corresponding aniline products (Table 4, entries 4–6).
Reusability is an important characteristic of the heterogeneous
catalysis which should be examined in catalytic reactions. The
(Table 6, entries 4 and 5) are converted to the corresponding alde-
hydes in good to high yields. Additionally, the secondary benzylic
alcohols are converted to the corresponding ketones in good yields
(Table 6, entries 6 and 8).
3 4
The reusability of Fe O @N-C@Pd Y-S (B) nanostructure was
recovery and reuse of the Fe
were examined. Therefore, we performed a reusability test for
the Fe @N-C@Pd Y-S (B) catalyst in the reduction of p-NP. After
the first run of the reduction, the catalyst could be efficiently and
easily recovered by external magnetic field and then washed with
3
O
4
@N-C@Pd Y-S (B) nanostructure
examined in aerobic oxidation of benzyl alcohol. It was found that
the recovery can be successfully achieved in ten successive reac-
tion runs (Table 7). We also examined the heterogeneous nature
of the catalyst was tested by ICP analysis and a hot filtration test.
3 4
O
3 4
ICP result of the used Fe O @N-C@Pd Y-S (B) nanostructure indi-
H
2
O and EtOH, and dried in air to reuse for the next run. The
cated the leaching of 3.5% of palladium after the ten cycles. Fur-
thermore, to ensure that the reaction was truly heterogeneous, a
hot filtration test (ꢀ50% conversion of benzyl alcohol) was per-
formed. The hot filtrates were then transferred to another flask
Fe O @N-C@Pd Y-S (B) nanostructure was recycled up to ten times
3 4
(Fig. 6a). The catalyst shows high catalytic activity with the k of
À1
0
.9554 min after 10th cycle, indicating the excellent reusability
of the catalyst. TEM image of the recovered catalyst in the 10th
cycle shows the retained Y-S structure and the absence the aggre-
gation of Pd NPs. (Fig. 6b). The heterogeneous nature of the catalyst
was tested by ICP analysis. The result indicated that the leaching of
3
containing PhCH (3 mL) at 90 °C. Upon further heating of the
catalyst-free solution for 12 h, no significant conversion (ꢀ1% by
GC analysis) was observed. These results confirmed the heteroge-
neous characteristic of the catalytically active species in this
reaction.
1
.3% of palladium after the ten cycles. Additionally, to ensure that
the reaction was truly heterogeneous, a filtration test (ꢀ50% con-
version of p-NP) was performed. The filtrates were transferred to
another flask. Upon further heating of the catalyst-free solution
for 100 min, no considerable progress (UV analysis) was observed.
These results confirmed the heterogeneous character of the cat-
alytically active species in this reaction.
4
. Conclusions
In this study we demonstrated for the first time the preparation
of Pd NPs supported on Fe
Fe @N-C@Pd C-S nanostructures were synthesized by two differ-
ent methods. The first method includes two steps: (i) the annealing
treatment of the Fe @PDA under a gas mixture of Ar/H flow and
ii) the decorating Pd NPs on the surface of annealed structures by
3 4
O @N-C C-S and Y-S nanostructures. The
3 4
O
A comparison was made of the present method with other
reported heterogeneous catalytic systems for the reduction of p-
2
O
3
2
NP and the results are presented in Table S2. The present method
(
À1
represents a simple, highly effective, and high TOF (980 h
)
sodium borohydride as a reducing reagent that led to the forma-
tion of the aggregate and small size of Pd NPs on the surface of
method for the reduction of p-NP.
the Fe
of Fe
treatment under H
3 4
O @N-C structure. The second method, the in-situ reduction
3
.3. The catalytic activity of Fe
3
O
4
@N-C@Pd Y-S (B) structure on the
2+
2+
2
O
3
and Pd in the Fe
O
2 3
@PDA@Pd structure by annealing
@N-C
oxidation of benzylic alcohols
2
flow as a reducing reagent led to Fe O
3 4
C-S nanostructure, which results to the no aggregate and medium
size of Pd NPs on the surface of C-S nanostructure. Additionally, the
HCl solution is used as an effective etching reagent to the partially
Aerobic oxidation of benzyl alcohol was selected as a model
reaction, and the effects of solvent, base, temperature, and amount
of catalyst were studied. The various solvents, such as PhCH
CH CN, and H O were tested (Table 5, entries 1–4). The best result
was obtained by using PhCH as a solvent in the model reaction
Table 5, entry 1). Among the various solvents studied, aprotic
3
, DMF,
3 4
etch the Fe O core to obtain Y-S nanostructures. The C-S and Y-S
3
2
nanostructures were used as the magnetic heterogeneous catalyst
in the reduction of nitroarenes and the oxidation of benzylic alco-
hols. The catalysts having large Pd particles exhibited high activi-
ties. It could be suggested that a different adsorption geometry of
the substrate on particles of different size could influence the rate
and the distribution products. Another explanation for this size
effect, it may be related to the increase of the hydrogen adsorption
with the Pd particle sizes. Additionally, the yolk-shell structures
showed high catalytic activity than the core-shell structures. This
might be to the nanorattle structure characteristics that indicated
the large free reaction voids inside the Y-S structures. The hetero-
geneous nature of the catalyst was proved using ICP analysis and a
hot filtration test.
3
(
3
polar solvents such as CH CN and DMF were found to be least
effective and provided poor conversion of benzyl alcohol to ben-
zaldehyde (Table 5, entries 2 and 3). This is probably due to the
coordination of the solvent molecules to the palladium metal,
resulting in poisoning and deactivation of the catalyst. The differ-
ent bases such as DBU, K
Table 5, entries 1 and 5–6). A superior yield obtained when
CO was used as the base. To study the effect of the amount of
the catalyst, the reactions were carried out at different amounts
of Fe @N-C@Pd Y-S (B) nanostructure ranging from 0.5 to 1.5
mol% of Pd and 1 mol% loading of Pd was found to be optimal
Table 5, entries 1 and 7–8). Additionally, as for the reaction tem-
perature, 90 °C was optimal (Table 5, entries 1 and 6–7). The other
synthesized catalysts (Fe @N-C@Pd C-S (A), Fe @N-C@Pd Y-S
A) and Fe @N-C@Pd C-S (B)) were used (Table 5, entries 11–
3). These results indicated that the order of the rate of the cat-
alytic activity was Fe @N-C@Pd Y-S (B) > Fe @N-C@Pd C-S
B) > Fe @N-C@Pd Y-S (A) > Fe @N-C@Pd C-S (A). The activity
2 3 3 4
CO , and K PO , were also screened
(
K
2
3
3 4
O
(
Notes
3
O
4
3 4
O
The authors declare no competing financial interest.
(
3 4
O
1
Acknowledgment
3
O
4
3 4
O
(
3
O
4
3 4
O
We gratefully acknowledge financial support from the Research
Council of Shahid Beheshti University and the Iranian National
Science Foundation (Proposal No: 94028845).
of the Pd catalysts generally increased with an increase in the Pd
particle size. A similar particle size effect has been reported for
the oxidation of cinnamyl alcohol over Pd/C catalyst [57]. As shown
in Table 5, the reaction can’t proceed in the absence of catalyst
(Table 5, entry 14).
Appendix A. Supplementary material
As evident from Table 6, under optimum reaction conditions,
various primary benzylic alcohols possessing both electron-
withdrawing (Table 6, entries 2 and 3) and donating substituents
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2018.05.003.

S.K. Movahed et al. / Journal of Catalysis 364 (2018) 69–79
79
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