Pd supported on g-C3N4 nanosheets: Mott-Schottky heterojunction catalyst for transfer hydrogenation of nitroarenes using formic acid as hydrogen source

Article abstract of DOI:10.1039/c7nj04268j

Well-dispersed Pd nanoparticles were anchored on g-C₃N₄ nanosheets, which were prepared by a liquid-phase exfoliation method. The hybrid material displayed excellent catalytic activity for the transfer hydrogenation of nitroarenes. The reactions can be conducted smoothly in water at room temperature with formic acid as hydrogen source. Aniline was produced in excellent yield (>99%; turnover frequency: 853), surpassing the majority of reported heterogeneous catalysts using formic acid or formates as hydrogen source. The increased activity of such a hybrid catalyst can be ascribed to the nitrogen-rich and amphipathic property of the carbon nitride support, the stable and uniform dispersed Pd nanoparticles and the Mott-Schottky effect between the g-C₃N₄ nanosheets and the metal nanoparticles. The heterogeneous Mott-Schottky catalyst can be reused for five cycles without any obvious loss of its catalytic activity.

Full text of DOI:10.1039/c7nj04268j

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Pd supported on gꢀC3N4 nanosheets: MottꢀSchottky
heterojunction catalyst for transfer hydrogenation of
nitroarenes using formic acid as hydrogen source
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a
a
Saisai Cheng , Xufeng Meng , Ningzhao Shang, Shutao Gao, Cheng Feng*, Chun Wang*, Zhi Wang
www.rsc.org/
College of Science, Hebei Agricultural University, Baoding 071001, China
Here, wellꢀdispersed Pd nanoparticles were anchored on gꢀC3N4 nanosheets, which were prepared by a liquidꢀ
phase exfoliation method. The hybrid material displayed excellent catalytic activity for the transfer
hydrogenation of nitroarenes. The reactions can be conducted smoothly in water at room temperature with
formic acid as hydrogen source. Aniline was produced in excellent yield (>99%, TOF: 853), surpassing
majority reported heterogeneous catalysts using formic acid or formates as hydrogen source. The boosting
activity of such a hybrid catalyst can be ascribed to the nitrogenꢀrich and amphipathic property of the carbon
nitride support, stable and uniform dispersed Pd nanoparticles and the MottꢀSchottky effect between the gꢀ
C3N4 nanosheets and the metal nanoparticles. The heterogeneous MottꢀSchottky catalyst can be reused for five
of cycles without any obvious loss of its catalytic activity.
Schottky catalysts for H2 generation reactions, hydrogenation
1. Introduction
3
reactions and carbonꢀcarbon coupling reactions,2, the activity of
which was remarkably enhanced by the varied electron density at the
interface of rectifying metalꢀsemiconductor contact. In the case, the
manner of activation of the carbon nitride support is analogous to the
function of a ligand in homogeneous catalysts, where the electron
density at the metal centers can be adjusted by introducing special
organic ligands with electronꢀdonating or ꢀaccepting groups.4 Mottꢀ
Schottky catalysts as heterogeneous version of metalꢀligand
complexes, the heterojunction of the metal nanoparticles with a
semiconductive support is strongly capable of changing the electron
density within the metal.5 As the work function of the electrons at
the Schottky barrier is continuous, and the Fermi levels adjust
through an effective flow of charges through the metalꢀsupport
interface.6, 7 In other words, electrons will flow until the Fermi level
on both sides of the interface is the same. Here, the role of the
support in heterogeneous catalysis is the same role taken by ligands
in homogeneous catalysis. Besides, this charging effect may be
much stronger than any ligand effect and finally enhances the
catalytic activity of the catalysts for specific catalytic reactions.4, 8
Studies on MottꢀSchottky catalysts based on graphitic carbon
nitride (gꢀC3N4) and Nꢀdoped carbon supported metal nanoparticles
for conventional catalysis, photocatalysis and electrochemical
catalysis is exponentially increasing.6 For example ， Li et al.9
engineered the interfacial structure of metalꢀmesoporous graphitic
carbon nitride (mpgꢀC3N4) catalyst for selective hydrogenation of
nitroarene compounds to aromatic amines at room temperature. The
Heterogeneous catalysts are preferred over homogeneous catalysts
systems for their ease of reusability and good compatibility with
flow reactors, accelerating the efficient production of various
chemicals.1 To boost the catalytic activity of heterogeneous
catalysts, much efforts have been paid on engineering the structure,
composition, morphology, defects and surface chemistry of the
catalysts.2 Besides, the supportꢀcatalyst phase interactions may also
dominate their catalytic performance. So rational regulating the
electron density within the metal via support effects is also a
straightforward strategy to enhance the activity of heterogeneous
metal catalysts.
Recently, Li et al. introduced
a
series of metal
naoparticles/carbon nitrideꢀbased semiconductive dyads as Mottꢀ
College of Sciences, Hebei Agricultural University, Baoding 071001, P. R. China.
Tel.: +86 312 7528291. Eꢀmail: chunwang69@126.com
（C.Wang）,
fengchengcctv@163.com
（C. Feng）
a These authors contributed equally to this work.
†Electronic Supplementary Information (ESI) available:
The MottꢀSchottky effect on the catalytic performance and the energyꢀdispersive Xꢀ
ray spectroscopy of the catalyst; Transmission electron microscope images, Pd
particleꢀsize distribution, Xꢀray Photoelectron Spectroscopy peaks of Pd and
powder Xꢀray diffraction patterns of recycled catalyst, TOF values of FA for
transfer hydrogenation of nitrobenzene over different catalysts, The kinetic curves
of the catalytic transfer hydrogenation of nitrobenzene at different reaction
temperatures. Reduction of nitrobenzene to aniline with different amount of formic
acid over the catalyst, Various reported catalyst tested for reduction of nitroarenes
into anilines. See DOI: 10.1039/x0xx00000x
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mpgꢀC3N4 was fabricated via a hard template approaches and the sustainability, transfer hydrogenation is the preferred methodologies
MottꢀSchottky catalyst need to be irradiated with light for a week to for the reduction.21 Although hydrogenation with H2 is the most
achieve excellent activity. Pd nanoparticles supported on atomꢀefficient approach, catalytic transfer hydrogenation also has
mesoporous gꢀC3N4 nanorods synthesized by nanocasting method advantages, as it avoids the use of high pressures, flammable H2 and
has also been prepared by the same group,10 the catalyst system can allows an easy operational setꢀup. Among the possible hydrogen
act a tandem catalyst to successively trigger the water reduction donors available for transfer hydrogenation, formic acid (FA) is
reaction to form H2 and activation of the asꢀformed H2 for further considered as a promising hydrogenꢀdonor owing to its low cost, low
reduction of 4ꢀnitrophenol to 4ꢀaminophenol. Chen et al.11 reported toxicity, high stability, and high energy density with a gravimetric
22
the application of the MottꢀSchottky catalyst based on gꢀC3N4 hydrogen content of 4.4 wt%.9,
More importantly, FA can be
supported Pd nanoparticles for hydrogenation of 2ꢀmethylfuran and manufactured in large quantities by the hydrogenation of carbon
other compounds with C=C bonds at roomꢀtemperature. The dioxide (CO2), biomass processing and artificial photosynthesis. So
reusability of such a hybrid catalyst is high due to the strong Mottꢀ far, FA has been employed as hydrogen donor in the catalytic
Schottky effect between the metal nanoparticles and the carbon transfer hydrogenation of nitro compounds,11 aldehydes,23 fructose
nitride support. The formation of MottꢀSchottky heterojunctions also to γꢀvalerolactone24 and alkene.25
affected the selectivity of the metal particles for some important
In this paper, gꢀC3N4 NS were prepared by a liquidꢀphase
catalytic reactions. Wang et al.12 reported that the Pd@mpgꢀC3N4 exfoliation method. A MottꢀSchottky heterojunction catalyst was
(polymeric mesoporous graphitic carbon nitride) catalyst exhibits a constructed by choosing gꢀC3N4 NS to support Pd nanoparticles for
high activity for the direct hydrogenation of phenol to the transfer hydrogenation of nitro compounds with FA as hydrogen
cyclohexanone, one of the more demanding reactions. The donor (Scheme 1).
hydrogenation proceeds efficiently with high selectivity and the
reaction could be even being performed under an atmospheric
2. Experimental section
pressure of hydrogen at 25 oC. Bu et al.13 prepared Ag/mpgꢀC3N4
Melamine, palladium (II), chloride (PdCl2, AR), sodium borohydride
heterojunction for RhB photodegradation. Bimetallic alloy PtCo
(NaBH4, 96%), nꢀDodecane (AR), and all of the nitro compounds
nanoparticles decorated 2D gꢀC3N4 (PtCo/gꢀC3N4) photocatalyst was
were obtained from Aladdin Chemicals Co Ltd. (Shanghai, China).
fabricated via an ethylene glycol reduction method.14 The 2D
Hydrofluoric acid (HF, 40%), Concentrated sulfuric acid (H2SO4,
nanosheets significantly enhanced the photocatalytic activity for H2
98%), Hydrochloric acid (HCl, 37%), acetone (AR), H2PdCl4
evolution, the photocatalytic performance of PtCo/gꢀC3N4
solution (2 mg mLꢀ1), ethyl acetate, Petroleum ether, formic acid
nanosheets was about 2.9 times higher than that of bulk Pt/gꢀC3N4.
(HCOOH, 88%), XCꢀ72R, SBAꢀ15 and anhydrous magnesium
The MottꢀSchottky catalyst of nitrogenꢀrich carbonꢀcoated cobalt
sulphate (MgSO4) were obtained from Chengxin Chemical Reagents
nanoparticles (Co@NC) was prepared and the Co@NC exhibited
Company (Baoding, China).
excellent activity for the baseꢀfree aerobic oxidation of alcohols to
esters.4 The enhancement of electron transfer at the metal/nitrogenꢀ
2.1 Synthesis of g-C3N4 nanosheet (g-C3N4 NS)
doped carbon interface is considered to be responsible for the gꢀC3N4 NS was prepared by the following method.26 Bulk gꢀC3N4
o
substantially enhanced activity of the catalyst. Despite recent was prepared by heating melamine in a muffle furnace at 550 C for
progress, design of efficient and sustainable MottꢀSchottkyꢀtype 4 h with a ramp rate of 2.3 oC minꢀ1. Porous gꢀC3N4 NS were
heterogeneous catalyst for organic synthesis, energy storage still synthesized via the chemical oxidation of bulk gꢀC3N4 with
remains in their infancy.
concentrated H2SO4. 3 g of the prepared bulk gꢀC3N4 was added to
Compared with bulk gꢀC3N4, mesoporous gꢀC3N4 and gꢀC3N4 the concentrated H2SO4 and stirred thoroughly for 2 h at 100 °C.
nanosheets (gꢀC3N4 NS)ꢀbased catalysts usually displayed higher Then the mixture was slowly poured into 100 mL of deionized water
(photo)catalytic activity due to the high surface area and thus and sonicated (1 h) for exfoliation. The obtained suspension was
enhance their adsorption capacity, the fast mass transfer of the then subjected to 10 min of centrifugation at 3000 rpm to remove
reactants and products to and from the catalytic sites.15 The existing any unexfoliated gꢀC3N4. After centrifugation at 8000 rpm, the
methods to obtain mesoporous gꢀC3N4 include soft template and hard obtained milkꢀwhite solid was washed thoroughly with deionized
template methods, However, both softꢀ and hardꢀtemplating methods water to remove the residual acid, and finally dried at 80 oC in oven
suffer from the shortcomings such as highꢀcost, timeꢀconsuming, overnight. The synthesis milkꢀwhite solid were named as gꢀC3N4 NS.
tedious synthesis processes and thus unfeasible for industrial
Synthesis of mpg-C3N4
application. gꢀC3N4 nanosheets can be obtained by acid exfoliation,
mpgꢀC3N4 material was prepared by the following method.27 In a
liquid ultrasonic exfoliation, chemical exfoliation, thermal oxidation
typical synthesis, melamine (5 g) was dissolved in a Ludox HS40
etching and so on.16 Among above strategies, acid exfoliation is
solution (7.5 g; dispersion of 14 nm SiO2 particles with 40 wt% in
actually the one in which atomic single layer gꢀC3N4 could be
water) and heated at 65 oC overnight to remove water. The white
obtained.17
powder was heated at a temperature of 600 oC for 4 h (ramp: 2.3 oC
Hydrogenation of nitroꢀcompounds to aromatic amines attracted
minꢀ1) under the protection of N2. The brownꢀyellow powder was
much attention owing to the wide applications of the latter molecules
recovered after dissolution of the silica framework in 4M HF acid,
in chemical and pharmaceutical industries.18ꢀ20 The most
filtration, several washings with ethanol, and drying at 60 oC under
straightforward approach for the synthesis of aniline and related
vacuum overnight.
compounds is the reduction of parent nitro compound. In terms of
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Scheme 1 Schematic illustration of the preparation process of the Pd@gꢀC3N4 NS catalyst.
speed of 300 rpm. The syntheses were performed either in a 45 mL
teflon jar with 5 mm in diameter ZrO2 balls (Changsha). Optical
Synthesis of boll-C3N4
The preparation process of bollꢀC3N4 as follows: 5 g yellow gꢀC3N4
powder was added into a teflon jar, followed by grinding with ZrO2
ball. After ball milling for 2 h, the obtained yellow gꢀC3N4 powder
was carefully collected.28
absorption measurement was taken with
a
UVꢀvisꢀNIR
spectrophotometer (UVꢀ3600, Shimadzu). The photoluminescent
emission spectra (PL) of the samples were recorded on a fluoromax
spectrometer (JASCOFPꢀ6500) by incident light of 318 nm at room
temperature. GC analyses were carried out on an Agilent 7820A
series gas chromatograph (Agilent Technologies, CA, USA)
equipped with a flame ionization detector (FID) and a split/splitless
injector. All the separations were performed on a HPꢀ5 capillary
column (30 m × 0.32 mm i.d. × 0.25 ꢁm film thickness) (Agilent
J&W Scientific, CA, USA). An Agilent 7820A gas chromatographyꢀ
5977E mass spectrometry system (GCꢀMS) (Santa Clara, CA, USA)
was employed to identify all reaction products.
2.2 Preparations of Pd@g-C3N4 NS, Pd@g-C3N4,
Pd@mpg-C3N4 and Pd@boll-C3N4
The gꢀC3N4 NS (100 mg) were dispersed into water (10 mL) by
sonication. 7.5 mL H2PdCl4 solution (2 mg mLꢀ1) was added into the
mixture and stirred for 2 h without the assistance of any surfactants
or other stabilizers. Then, 0.1M NaBH4 (10 mL) was added
dropwise to this suspension at 25 oC. After 2 h, the obtained mixture
was separated by centrifugation and washed thoroughly with
distilled water and ethanol, then dried at 60 oC for 6 h. The resultant
2.4 Catalytic transfer hydrogenation of nitro compounds
material, Pd@gꢀC3N4 NS, was characterized using different Typically, nitro compound (1 mmol) and catalyst (10 mg) were
techniques and applied as catalyst for transfer hydrogenation of mixed with distilled water (5 mL) by sonication for 2 min in a round
nitroarene compounds. Pd@gꢀC3N4, Pd@mpgꢀC3N4 and Pd@bollꢀ bottle (25 mL), after which 215 ꢁL formic acid (5 mmol) was added
C3N4 were prepared with the above procedure except that gꢀC3N4, to the solution and then the reaction mixture was stirred at 25 oC for
mpgꢀC3N4 and bollꢀC3N4 were used, respectively.
a certain time. The reaction progress was monitored by thin layered
chromatography (TLC) and gas chromatography 14. The conversion
and selectivity were calculated from the GC data. After completion
of the reaction, the recovered catalyst was separated by
centrifugation and washed with water and methanol to remove the
2.3 Characterization of samples
Xꢀray photoelectron spectroscopy (XPS) spectra were acquired with
an ESCA Lab 250 (Thermo) spectrometer (Shanghai, China) using a
monochromatic Al Ka source (1486.6 eV). The Xꢀray diffraction
(XRD) patterns of the samples were recorded with a Rigaku D/max
2500 Xꢀray diffractometer (Dandong, China) using Cu Kα radiation
(30 kV, 25 mA) in the range 2θ = 2°ꢀ80°. Fourier transformed
infrared (FTIR) spectra were recorded on a Bruker VERTEX 700
spectrometer between 4000 and 500 cmꢀ1. The size and morphology
of the nanoparticles were observed by scanning electron microscopy
o
inorganic and organic part, then dried in vacuum at 60 C overnight
for another consecutive reaction run. The product was extracted with
ethyl acetate (3×5 mL) and dried over anhydrous MgSO4. The
identification of the products was confirmed by GCꢀMS.
3. Results and Discussion
(SEM) using a Hitachi S4800 field emission electron microscope gꢀC3N4 nanosheets were prepared via the chemical oxidation
(Tokyo, Japan) operated at 30 kV and transmission electron exfoliation of bulk gꢀC3N4 with concentrated H2SO4. Wellꢀdispersed
microscopy (TEM) using a JEOL model JEMꢀ201123 (Tokyo, Japan) Pd nanoparticles were anchored on gꢀC3N4 NS by an impregnation
at 200 kV. Atomic force microscope (AFM) images were acquired in method coupled with subsequent reduction with NaBH4.
phase mode in air using a Digital Instrument Shimadzu SPMꢀ9600
3.1 Characterization of the catalyst
(The samples were prepared by dropꢀcasting corresponding dilute
The microstructural features of the prepared samples were analyzed
dispersions onto freshly cleaved mica surface). The Brunauerꢀ
using transmission electron microscopy (TEM) micrographs (Fig. 1).
EmmettꢀTeller (BET) surface areas were determined from the N2
The TEM images display the ultrathin layers of gꢀC3N4 NS, which
adsorption at 77 K using VꢀSorb 2800P (Jinaipu, China). The metal
provide clear evidence that the bulk gꢀC3N4 was exfoliated to
content of the materials was analyzed by a T.J.A. ICPꢀ9000 type
distinguishable layers. The TEM images reveal wellꢀdispersed Pd
inductively coupled plasma atomic emission spectroscopy (ICPꢀAES)
nanoparticles on the surface of gꢀC3N4 NS (Fig. 1A), and display
instrument. Ball mill syntheses were carried out in a Fritsch
clear lattice fringe, suggesting the good crystallinity (Fig. 1B). Fig.
Pulverisette 7 premium line planetary ball mill operating at a rotation
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1B shows crystal planes of Pd, and the crystal plane spacings were
In addition, the Xꢀray diffraction (XRD) results also coincide
measured as 0.224 nm. Besides, a lattice spacing of 0.33 nm was with the TEM images, further supporting the successful attachment
observed, which is attributed to the (002) plane of gꢀC3N4. We also of Pd particles on gꢀC3N4 NS. The XRD patterns of gꢀC3N4 NS and
observed that there is a good interface contact between metals and bulk gꢀC3N4 materials are displayed in Fig. 2A. As seen, two
the material, which illustrates the existence of the heterojunction in diffraction peaks located at 2θ = 13.1° and 27.4°, are present in the
the hybrid material. The HRTEM observation indicates that the diffractograms. The main peak (at 2θ = 27.4°) is ascribed to the (002)
average size of Pd particles in the Pd@gꢀC3N4 NS samples was planes, typical interlayer stacking reflection of conjugated aromatic
approximately 3.56 nm on the basis of a count of more than 100 systems. The minor peaks (2θ = 13.1°), correspond to a distance of
individual nanoparticles via TEM analysis (Fig. 1E). The content of 0.675 nm and was indexed as (100) planes inꢀplanar trisꢀsꢀtriazine
Pd in the Pd@gꢀC3N4 NS samples was 7.48 wt%, as demonstrated structural packing motifs 29. It is apparent that there is no evident
by ICP analysis. No obvious agglomeration of the Pd nanoparticles different in XRD patterns between the nanosheets and bulk materials,
was observed in the TEM images, revealing that the inbuilt indicating that the chemical treatment with concentrated H2SO4 does
functional nitrogen groups in the gꢀC3N4 NS firmly anchor metal not destroy the structure of carbon nitrides. The broaden peak of Pd
nanoparticles. However, the Pd particles supported on bulk gꢀC3N4 (111) of Pd@gꢀC3N4 NS sample at 40.1° can be ascribed to the high
(Pd@gꢀC3N4) material emerge slight agglomeration phenomenon dispersion of Pd nanoparticles on gꢀC3N4 NS. The gꢀC3N4 NS
and gꢀC3N4 presented a bulk state (Fig. 1C, 1D). The mean size of exhibits the specific surface area of 14.2 m2gꢀ1, nearly two times
Pd nanoparticles in the Pd@gꢀC3N4 samples was approximately 6.93 larger than that of pristine gꢀC3N4 (8.3 m2gꢀ1) (Fig. 2B). Anyway, the
nm (Fig. 1F), which is about 2 fold higher than that of Pd@gꢀC3N4 high specific surface area of the gꢀC3N4 NS is benefit for absorbing
NS samples. Those TEM results strongly reveal the exfoliation of
more active species and reactants on the surface, which may result in
a better catalytic activity30. The lower surface area of Pd@gꢀC3N4
NS (12.8 m2gꢀ1) can be ascribed to the fact that a certain amount of
Pd nanoparticles were embedded in the surface of gꢀC3N4 NS during
the fabrication process.
B
A
C
D
Fig. 2 XRD pattern of bulk gꢀC3N4, gꢀC3N4 NS, Pd@gꢀC3N4, Pd@gꢀ
C3N4 NS (A) and the nitrogen adsorptionꢀdesorption isotherms of
bulk gꢀC3N4, gꢀC3N4 NS, Pd@gꢀC3N4, Pd@gꢀC3N4 NS (B).
Fig. 3 FTꢀIR spectra of bulk gꢀC3N4 and gꢀC3N4 nanosheet samples
(A) and Pd XPS spectra of Pd@gꢀC3N4 NS (B).
Fig. 1 the TEM images of the synthesized Pd@gꢀC3N4 NS (A, B)
and Pd particleꢀsize distribution (E) in the Pd@gꢀC3N4 NS, the TEM
images of the synthesized Pd@gꢀC3N4 (C, D) and Pd particleꢀsize
distribution (F) in the Pd@gꢀC3N4.
Fig. 3A show the FTꢀIR spectra of bulk gꢀC3N4 and gꢀC3N4 NS
samples, respectively. For gꢀC3N4, the broad bands in the 3500ꢀ2800
cmꢀ1 region can be assigned to NꢀH stretching vibration modes. The
marked bands located at 1000ꢀ1650 cmꢀ1 are due to the stretching
vibration modes of C=N and CꢀN heterocycles. Additionally, the
significant band appearing at 803 cmꢀ1 is attributed to the ring
sextant outꢀofꢀplane bending vibration of triꢀsꢀtriazine units. Besides,
the FTIR spectra verify that the structure of gꢀC3N4 stays unchanged
C3N4 from bulk form to nanosheets can facilitate the dispersion of
the metal particles. Furthermore, the elemental mapping reveals that
the Pd atoms are uniformly distributed in the Pd@gꢀC3N4 NS
samples (Fig. S2A) and the energy spectrum diagram further shows
the presence of Pd and N (Fig. S2B).
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after prepared by liquidꢀphase exfoliation method compared with the
Moreover, the component of the underlying support also plays
original graphitic structure of gꢀC3N4.29 The Pd Xꢀray photoelectron an essential role in determining the catalytic performance of the final
spectroscopy (XPS) of the catalyst demonstrates
a doublet materials, and was displayed in the following part. For comparison,
corresponding to Pd 3d5/2 and Pd 3d3/2 (Fig. 3B). The Pd 3d5/2 peak mpgꢀC3N4 and bollꢀC3N4 were prepared (See ESI). The transfer
and Pd 3d3/2 peak at 336.1 eV and 341.3 eV are attributed to Pd0 hydrogenation of nitrobenzene over Pd@gꢀC3N4, Pd@mpgꢀC3N4,
(metallic palladium), while the Pd 3d5/2 peak and Pd 3d3/2 peak at Pd@bollꢀC3N4 and commercially available Pd@C were investigated
338 eV 343.1 eV is related to Pd2+ (palladium oxide).12 According to under the same reaction conditions (Fig. S4). Surprisingly, the TOF
the XPS data, Pd0 were formed as the major phase on the surface of value of the Pd@gꢀC3N4 NS catalyst was up to 693 mol molꢀ1 Pd hꢀ1
the support.
at 298 K, surpassing other catalysts described herein. Those results
Theoretical investigation suggested that the type of nitrogenꢀ exhibited that MottꢀSchottky heterojunction in catalyst plays very
rich carbon, as ambipolar support acting in the metal/NC important role for the catalytic transfer hydrogenation of nitroarenes.
heterojunction, is mainly dominated by the work functions of two
Furthermore, optimal quantities of reducing agent were found by
components. In this work, the work function of Pd is located close to gradually changing the amount of formic acid (Table S1). In the
that of gꢀC3N4, the flow of electrons (Fig. S1C) sensitively depends presence of 3 equiv and 1 equiv amounts of FA, the yields of aniline
on the nitrogen content and the structure. The Schottky barrier will after 30 min were 84% and 62%, respectively. The results indicated
apparently result in electron redistribution at the interface of Pd and that 5 equiv FA is enough for the transfer hydrogenation of
gꢀC3N4 and enrich the positive charges on the side of metallic Pd. nitrobenzene; the reaction can be completed after 10 min at 25 oC in
Typical XPS peaks of pyridinic nitrogen in Pd@gꢀC3N4 NS a high yield of 99% with excellent selectivity. The influence of the
gradually shift to higher binding energy compared with reported data dosage of catalyst was investigated also. When the dosage of Pd@gꢀ
7
of Pd@gꢀC3N4 (Fig. S1A),4, which indicated that the electron C3N4 NS catalyst was 2, 5, 10 and 13 mg, the reaction time needed
density on the nitrogen decreased and the electron transferred over 60, 30, 10 and 5 min, respectively, to achieve complete conversion
the MottꢀSchottky heterojunction to the metallic Pd.
(Table 1, entries 7ꢀ10). The maximum TOF value was estimated to
Compared with commercial Pd@XCꢀ72R, the increased XPS be 1313 mol molꢀ1 Pd hꢀ1 (Table 1, entry 10), surpassing majority
peak intensity of Pd in the Pd@gꢀC3N4 NS may be caused by the heterogeneous catalysts using FA or formates as hydrogen source for
possible presence of oxo functional groups in the gꢀC3N4 NS (Fig. transfer hydrogenation of nitrobenzene (Table S2). These results
S1B).17 Besides, typical XPS peaks of Pd in Pd@gꢀC3N4 NS illustrated the excellent activity of such
a
MottꢀSchottky
gradually shifted to lower binding energy compared with reported heterojunction catalyst for promoting the hydrogenation of
data for Pd@mpgꢀC3N4 (Fig. S1B).12 The fact that the electron nitrobenzene reactions.
density at the Pd increases further illustrates electron transfer over
the MottꢀSchottky heterojunction at the interface.
The effect of reaction solvent (MeOH, EtOH, H2O and EtOHꢀ
H2O) on the transfer hydrogenation reaction was also investigated.
After the introduction of Pd nanoparticles, the Using H2O, MeOHꢀH2O and EtOHꢀH2O (1:1, v/v) as the solvent, the
photoluminescence intensity of gꢀC3N4 NS decreased significantly, desired product was obtained in 99% yield after 10 min (Table 1,
speaking for an effective electron transfer from gꢀC3N4 NS to Pd entries 9, 11ꢀ12). Water possesses the advantages of environmental
nanoparticles (Fig. 4).2
friendly and low cost, so water was chosen as the reaction solvent.
Fig. 5 Stability test for the transfer hydrogenation in the presence of
o
Fig. 4 UVꢀvis absorbance spectra (A) and photoluminescence Pd@gꢀC3N4 NS catalyst at 25 C (A) and the filtration test for the
spectra (B) of gꢀC3N4 NS and Pd@gꢀC3N4 NS.
reduction of nitrobenzene over Pd@gꢀC3N4 NS (B).
3.2 The catalytic hydrogenation of nitroarenes
The reusability and stability of a catalyst is crucial for its practical
We initially tested the possibility of transfer hydrogenation of application, so the catalytic stability of Pd@gꢀC3N4 NS was further
nitrobenzene at 25 oC with water as the solvent and FA as the only evaluated. The catalyst was recovered by centrifugation and washed
hydrogen source. For a blank test, no conversion was observed in the with water and ethanol, then dried at 60 oC under vacuum. The
absence of catalyst, or noble metals Pd nanoparticles (Table 1, recovered catalyst was found to retain high activity with only a
entries 1ꢀ3), suggesting that Pd nanoparticles play a key role in minor decline in catalytic performance (96% conversion at the fifth
catalyzing the hydrogenation reaction.
cycle) after recycled five times under the same reaction conditions,
which confirm the robustness of the catalyst (Fig. 5A). The TEM
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images of the used catalyst (Fig. S3A) confirm that the Pd have no obvious difference with that of the fresh catalyst, declaring
nanoparticles supported on gꢀC3N4 NS remained good dispersion and the stability of the MottꢀSchottky heterojunction catalyst. In
no significant particle agglomeration (4.18 nm) after five cycles (Fig. summary, the reusability of the hybrid catalyst is excellent due to the
S3B). In addition, XRD results indicated that no obvious diffraction strong MottꢀSchottky effect between the carbon nitride support and
peak change can be observed between fresh and recycled catalyst the metal nanoparticles and the surface functional groups on gꢀC3N4
(Fig. S3D), indicating good dispersion of metal and high durability NS.
and stability of the hybrid material. The fact that the concentration of
In order to further verify the heterogeneity of the Pd@gꢀC3N4
leached palladium concentration in the filtrate after reaction was NS catalyst, a filtration test was carried out after 5 min reaction. As
below the detection level (0.1 ppm) of the ICPꢀMS suggests that the shown in Fig. 5B, no further conversion of nitrobenzene was
catalytic reactions proceeded heterogeneously, which can be observed even after 1 h of reaction under identical conditions.
attributed to the presence of the amino groups on the surface of gꢀ According to the above results, it can be concluded that the Pd@gꢀ
C3N4 NS that act as “anchors” for the metal Pd nanoparticles, C3N4 NS possesses a truly heterogeneous nature and presents
preventing agglomeration and leaching during the hydrogenation impressive catalytic activity and recyclability, showing a good
progress. Moreover, the XPS spectra of the used catalyst (Fig. S3C) potential application in the chemical industry.
Table 1 Reduction of nitrobenzene to aniline under various conditions. [a]
Catalyst
NO2
NH2
FA
Entry
Catalysts
Solvents
Dosage (mg)
Time (min)
Temperature (oC)
Conversion (%)
Selectivity (%)
TOF
1
2
ꢀ
Water
Water
2
2
6 h
6 h
6 h
6 h
6 h
3 h
60
30
10
5
25
25
25
25
50
25
25
25
25
25
25
25
ꢀ
ꢀ
ꢀ
ꢀ
gꢀC3N4 NS
ꢀ
ꢀ
3
gꢀC3N4
Water
2
ꢀ
ꢀ
ꢀ
4
Pd@gꢀC3N4
Pd@gꢀC3N4
Pd@C
Water
2
7
96
8
5
Water
2
21
67
97
100
100
100
100
100
83
23
6
Water
17
2
96
3
7
Pd@gꢀC3N4 NS
Pd@gꢀC3N4 NS
Pd@gꢀC3N4 NS
Pd@gꢀC3N4 NS
Pd@gꢀC3N4 NS
Pd@gꢀC3N4 NS
Water
>99
>99
>99
>99
>99
>99
693
569
853
1313
853
853
8
Water
5
9
Water
10
13
10
10
10
11
12
Water
Methanol:Water(1:1)
Ethanol:Water(1:1)
10
10
[a] reaction condition: nitrobenzene (1 mmol), FA (5 mmol), catalyst (7.48 wt% Pd loading Pd@gꢀC3N4 NS), solvent (5 mL); GC analysis
using nꢀdecane as an internal standard.
activation energy, thus facilitating the reaction at lower temperatures
and saving energy.31
3.3 Kinetic study of the transfer hydrogenation of nitrobenzene
Ea
Kinetic study was then studied for the catalytic transfer
RT
k
=
Ae−
(2)
hydrogenation of nitrobenzene over Pd@gꢀC3N4 NS catalyst. The
o
o
data of kinetic study were collected at 0 C, 15 oC and 25 C under
ambient pressure due to the high catalytic activity of the Pd@C3N4
NS catalyst. The apparent rate constant (k) for the pseudo first order
reaction could be defined as the following equation (1), where Ct is
Ea
1
ln k =
+ ln A (3)
(−
)
R
T
To demonstrate the general applicability of this catalyst system,
the concentration of nitrobenzene that changes during the time; C0 is various substituted nitro compounds were investigated under the
the initial concentration of nitrobenzene; k is the rate constant and t optimized conditions (Table 2). The reaction was carried out in
is the reaction time.
water with FA as hydrogen donor at room temperature. Alkylꢀ
substituted nitroarenes are reduced rapidly in excellent yields up to
99% (Table 2, entries 2, 3), but a prolonged time is needed when the
nitro group is sterically hindered. Moreover, the nitro compounds
Ct
ln
= ꢀk t (1)
C0
As shown in Fig. S5 the rate constant k increases with the rise involving electronꢀrich groups such as oꢀhydroxyl, pꢀhydroxyl, pꢀ
of temperature. According to the Arrhenius equation (Eq. (2)), the amino, pꢀhydroxymethyl and pꢀmethoxy were reduced to
apparent activation energies of the catalytic hydrogenation of Pd corresponding amine products in an efficient fashion (Table 2,
over gꢀC3N4 NS is calculated to be 40 kJ·molꢀ1, indicating that entries 4ꢀ8). Interestingly, for the Clꢀsubstituted nitroarenes, obvious
combination of Pd nanoparticles over gꢀC3N4 NS greatly reduces the dehalogenation processes are observed during the reduction, which
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give the aniline products in 100% (Table 2, entry 9). However, for weak. Clꢀ is easy to exchange with HCOOꢀ, which makes the
Brꢀsubstituted nitroarenes no obvious dehalogenation processes are dechlorination easy. After having demonstrated successfully the
observed during the reduction, which give the aniline products in reduction of halogenated nitroarenes, we applied the procedure for
93% yield (Table 2, entry 10). According to the theory of hardꢀsoftꢀ the transfer hydrogenation of demanding functionalized nitroarenes.
acidꢀbase, Brꢀ is a soft base and Pd [O] is a soft acid, which result in Interestingly, the Pd@gꢀC3N4 NS catalyst allows for the selective
a stronger binding force between Br and Pd in PhPdBr produced in transfer hydrogenation of nitro group in the presence of ketone group
the oxidation addition reaction. So in the progress of transfer (Table 2, entries 11ꢀ14). For the hydrogenation of oꢀ
hydrogenation dehalogenation, Brꢀ is not easy to exchange with nitroacetophenone, the yield of the product is only 84% even after 5
HCOOꢀ, making it difficult to conduct hydrodehalogenation reaction. h.
However, Clꢀ is a hard base, the binding force between Cl and Pd is
Table 2 Reduction of nitro compounds to corresponding amines. [a]
Cat, FA
R
NH2
R
NO2
Water, 25 o
C
Time
(min)
Conversion
(%)
Selectivity
Entry
Substrate
Product
TOF
TON
(%)
1
2
10
20
100
>99
>99
853
427
142
142
NO2
>99
>99
>99
3
4
35
10
>99
>99
244
853
142
142
5
15
>99
>99
569
142
H2N
NO2
6
7
15
>99
>99
>99
>99
569
57
142
142
2.5 h
8
9
60
60
2h
>99
>99
93
>99
>99
95
142
142
63
142
142
126
10[b]
11
12
30
45
>99
>99
>99
>99
284
190
142
142
13
5 h
60
84
>99
>99
24
119
142
14[c]
>99
142
15
16
3 h
30
>99
>99
>99
95
47
142
135
270
[a] Substrate (1 mmol), FA (5 mmol), 10 mg of Pd@gꢀC3N4 NS, water (5 mL), 25 oC; GC analysis using nꢀdecane as an internal standard.
[b]50 oC, HCOOH : HCOONH4 (5 mmol : 5 mmol). [c] 50 oC.
The reaction of oꢀnitroacetophenone can be completed within that both the double bond and nitro group were reduced (Table 2,
60 min when the reaction temperature rose to 50 oC, which revealed entry 16).
that obviously space steric hindrance to the hydrogenation of the
nitro group can be tolerated. The reduction of 1ꢀnitronaphthalene to
the corresponding amines with excellent hydrogenation performance
4. Conclusions
was also achieved (Table 2, entry 15). Next, the reduction of 4ꢀ In summary, gꢀC3N4 NS were successfully prepared by liquidꢀphase
nitrostyrene containing both olefin and nitro moieties was further exfoliation method. The asꢀobtained gꢀC3N4 NS can function as an
investigated using Pd@gꢀC3N4 NS as catalyst. The result showed effective and robust support for chaperoning Pd nanoparticles. The
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hybrid material showed high catalytic activity and stability in the
hydrogenation of nitro compounds. The maximum turnover
frequency (TOF) of the catalyst was estimated to be 1313 mol molꢀ1
Pd hꢀ1. The charge equilibration between the metal and support plays
an important role in dictating the overall energetic of the composite
in the contact interface. Considering high performance stability in
various polar solvents, high nitrogen content and high reactant
compatibility, gꢀC3N4 NS might be widely applicable as a supporter
for integrating different functional metal particles into
multifunctional hybrid materials for more complicated reactions with
novel properties and extended applications. Meanwhile, exploring
the catalytic activity of those composites can pave the way for
designing novel catalytic systems.
10. X.ꢀH. Li, X. Wang and M. Antonietti, Chem. Sci., 2012, 3,
2170.
11. L.ꢀH. Gong, Y.ꢀY. Cai, X.ꢀH. Li, Y.ꢀN. Zhang, J. Su and J.ꢀS.
Chen, Green Chem., 2014, 16, 3746ꢀ3751.
12. Y. Wang, J. Yao, H. Li, D. Su and M. Antonietti, J. Am. Chem.
Soc., 2011, 133, 2362ꢀ2365.
13. Y. Bu, Z. Chen and W. Li, Appl Catal B: Environ., 2014, 144
,
622ꢀ630.
14. C. Han, Y. Lu, J. Zhang, L. Ge, Y. Li, C. Chen, Y. Xin, L. Wu
and S. Fang, J. Mater. Chem. A, 2015, , 23274ꢀ23282.
3
5. Acknowledgements
15. J. Wen, J. Xie, X. Chen and X. Li, Appl. Surf. Sci., 2017, 391
,
Financial supports from the National Natural Science Foundation of
China (21603054) the Natural Science Foundation of Hebei Province
(B2015204003, B2016204131), the Young Topꢀnotch Talents
Foundation of Hebei Provincial Universities (BJ2016027), and the
Natural Science Foundation of Hebei Agricultural University
(ZD201506, ZD201613) are gratefully acknowledged.
72ꢀ123.
16. L. Wang, C. Wang, X. Hu, H. Xue and H. Pang, Chem. Asian.
J, 2016, 11, 3305ꢀ3328.
17. A. Shi, H. Li, S. Yin, B. Liu, J. Zhang and Y. Wang, Appl Catal
B: Environ., 2017, 218, 137ꢀ146.
Notes and references
18. A. M. Tafesh and J. Weiguny, Chem. Rev., 1996, 96, 2035.
19. R. Bal, A. Shukla, R. K. Singha and T. Sasaki, Green Chem.,
2015, 17, 785ꢀ790.
College of Science
Hebei Agricultural University
Baoding 071001, Hebei Province, P. R. China
Fax: (+86)312ꢀ7528292
20. R. S. Downing, P. J. Kunkeler and H. V. Bekkum, Catal.
Today, 1997, 37, 121ꢀ136.
Eꢀmail: chunwang69@126.com (C. Wang);
fengchengcctv@163.com (C. Feng)
21. M. J. Gilkey and B. Xu, ACS Catal., 2016,
22. R. V. Jagadeesh, K. Natte, H. Junge and M. Beller, ACS Catal.,
2015, , 1526ꢀ1529.
23. J. R. CabreroꢀAntonino, R. Adam, K. Junge, R. Jackstell and
M. Beller, Catal. Sci. Technol., 2017, , 1981ꢀ1985.
24. L. Qi and I. T. Horváth, ACS Catal., 2012, , 2247ꢀ2249.
25. J. Mondal, Q. T. Trinh, A. Jana, W. K. Ng, P. Borah, H. Hirao
6, 1420ꢀ1436.
1. A. Molnar, Chem. Rev., 2011, 111, 2251ꢀ2320.
2. X.ꢀH. Li, M. Baar, S. Blechert and M. Antonietti, Scientific
5
Reports, 2013, 3.
3. Y. Y. Cai, X. H. Li, Y. N. Zhang, X. Wei, K. X. Wang and J. S.
Chen, Angew. Chem. Int. Ed. Engl., 2013, 52, 11822ꢀ11825.
4. H. Su, K. X. Zhang, B. Zhang, H. H. Wang, Q. Y. Yu, X. H. Li,
7
2
M. Antonietti and J. S. Chen, J. Am. Chem. Soc., 2017, 139
,
and Y. Zhao, ACS Appl. Mater. Interfaces, 2016, 8, 15307ꢀ
811ꢀ818.
15319.
5. H. Q. Xu, J. Hu, D. Wang, Z. Li, Q. Zhang, Y. Luo, S. H. Yu
and H. L. Jiang, J. Am. Chem. Soc., 2015, 137, 13440ꢀ13443.
6. X. H. Li and M. Antonietti, Chem. Soc. Rev., 2013, 42, 6593ꢀ
6604.
26. Z. Zhou, J. Wang, J. Yu, Y. Shen, Y. Li, A. Liu, S. Liu and Y.
Zhang, J. Am. Chem. Soc., 2015, 137, 2179ꢀ2182.
27. K. K. Datta, B. V. Reddy, K. Ariga and A. Vinu, Angew. Chem.
Int. Ed., 2010, 49, 5961ꢀ5965.
7. J. Manuela and L. Haim, Nano Lett., 2003, 3, 353ꢀ358.
28. C. Teng, D. Xie, J. Wang, Z. Yang, G. Ren and Y. Zhu, Adv.
Funct. Mater., 2017, 27, 1700240.
8. X. Li, J. Zhang and X. W. Wang, Chem. Mater., 2011, 23
,
4344–4348.
29. Y. Wu, M. Wen, M. NavlaniꢀGarcia, Y. Kuwahara, K. Mori
and H. Yamashita, Chem. Asian. J, 2017, 12, 860ꢀ867.
9. X. H. Li, Y. Y. Cai, L. H. Gong, W. Fu, K. X. Wang, H. L.
Bao, X. Wei and J. S. Chen, Chemistry, 2014, 20, 16732ꢀ16737.
8 | J. Name., 2012, 00, 1-3
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Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 9 of 10
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DOI: 10.1039/C7NJ04268J
Journal Name
30. C. Ye, J.ꢀX. Li, Z.ꢀJ. Li, X.ꢀB. Li, X.ꢀB. Fan, L.ꢀP. Zhang, B.
ARTICLE
Chen, C.ꢀH. Tung and L.ꢀZ. Wu, ACS Catal., 2015, 5, 6973ꢀ
6979.
31. J. Xia, G. He, L. Zhang, X. Sun and X. Wang, Appl Catal B:
Environ., 2016, 180, 408ꢀ415.
This journal is © The Royal Society of Chemistry 20xx
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Pd supported on g-C3N4 nanosheets: Mott-Schottky
heterojunction catalyst for transfer hydrogenation of
nitroarenes using formic acid as hydrogen source
Saisai Cheng, Xufeng Meng, Ningzhao Shang, Shutao Gao, Cheng Feng*, Chun
Wang*, Zhi Wang
College of Science, Hebei Agricultural University, Baoding 071001, China
Well-dispersed Pd nanoparticles anchored on g-C3N4 nanosheets displayed excellent
catalytic activity for the transfer hydrogenation of nitroarenes.