Pd-doped Ni nanoparticle-modified N-doped carbon nanocatalyst with high Pd atom utilization for the transfer hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c7gc03710d

Palladium (Pd)-based catalysts with maximum utilization of the Pd atoms are attractive for hydrogenation reactions and conserving Pd resources. Herein, the highly dispersed Ni nanoparticle (NP)-modified mesoporous N-doped carbon (Ni/mCN) was successfully prepared by pyrolyzing a mixture of polyacrylonitrile, melamine and Ni(NO₃)₂·6H₂O. Then, the resulting Ni/mCN material with highly dispersed metallic Ni NPs was treated with Pd(AcO)₂, and Pd²⁺ was spontaneously reduced to metallic Pd by the Ni NPs, affording the PdNi NP-based catalyst (PdNi/mCN). The spontaneous reduction process deposits most of the Pd atoms on the surface of the Ni NPs, thus allowing for the maximum utilization of the noble metal Pd. The prepared mesoporous N-doped carbon support can not only provide more surface area to adsorb reaction substrates, but also enhances the accessibility of the active sites of PdNi NPs. The prepared PdNi/mCN nanocatalyst shows a very high catalytic activity for the transfer hydrogenation of nitroarenes using formic acid as the reductant under ambient conditions in aqueous solution, as compared to other Pd-based catalysts, probably because of the highly dispersed PdNi NPs and the maximum utilization of the Pd atoms, as well as the superior structure of mCN. Moreover, the PdNi/mCN nanocatalyst exhibits excellent recyclability and reusability, and the catalytic activity does not obviously decrease after ten reaction cycles. Therefore, we believe that this study should open a new frontier in the preparation of porous N-doped carbon-supported catalysts with maximum utilization of the noble metals for green and sustainable catalysis.

Full text of DOI:10.1039/c7gc03710d

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Pd doped Ni nanoparticles modified N-doped carbon nanocatalyst with high Pd
atom utilization for transfer hydrogenation of nitroarenes
Xueliang Cui, Yu Long, Xia Zhou, Guiqin Yu, Jin Yang, Man Yuan, Jiantai Ma and Zhengping
Dong*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Gansu Provincial Engineering Laboratory for Chemical Catalysis, Lanzhou
University, Lanzhou 730000, PR China.
Eꢀmail: dongzhp@lzu.edu.cn (Zhengping Dong)
Fax: +86 931 8912582; Tel: +86 931 8912577
Abstract
Palladium (Pd) based catalysts with maximum utilization of Pd atoms is attractive for
hydrogenation reactions and saving the Pd resource. Herein, highly dispersed Ni nanoparticles
(NPs) modified mesoporous Nꢀdoped carbon (Ni/mCN) was first successfully prepared via
pyrolyzing a mixture of polyacrylonitrile, melamine, and Ni(NO3)2·6H2O. Then, the resulting
Ni/mCN material with highly dispersed metallic Ni NPs was treated with Pd(AcO)2, and Pd2+
spontaneously reduced to metallic Pd by Ni NPs, affording PdNi NPs based catalyst
(PdNi/mCN). The spontaneous reduction process deposits most of the Pd atoms on the
surface of Ni NPs, thus allowing maximum utilization of noble metal Pd. The prepared
mesoporous Nꢀdoped carbon support can not only provide more surface area to adsorb
reaction substrates, but also enhance the accessibility of the active sites of PdNi NPs. The
prepared PdNi/mCN nanocatalyst shows a very high catalytic activity for transfer
hydrogenation of nitroarenes using formic acid as the reductant under ambient conditions in
aqueous solution as compared with other Pd based catalysts, probably because of the highly
dispersed PdNi NPs and the maximum utilization of Pd atoms as well as the superior structure
of mCN. Moreover, the PdNi/mCN nanocatalyst exhibits excellent recyclability and
reusability, and catalytic activity does not obviously decrease after ten reaction cycles.
Therefore, we believe that this study should open a new frontier on preparing porous Nꢀdoped
carbon supported catalysts with maximum utilization of noble metals for green and
sustainable catalysis.
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Introduction
Palladium (Pd) is one of the most common metals used to catalyze chemical reactions such
as oxidation,1, 2 hydrogenation,3, 4 hydrogenation decomposition,5ꢀ7 carbon–carbon coupling8
and electrochemical reactions.9, 10 Small Pd nanoparticles (NPs) exhibit large specific surface
area, thus can expose more surface Pd atoms and have been investigated as robust catalysts
for catalyzing chemical reactions.11 However, heterogeneous catalytic reaction usually only
occurs on the surface of the metal NPs, most of metal atoms in the core do not exhibit any
catalytic activity. For example, according to a model of the fullꢀshell “magic number” clusters,
in a 5ꢀnm closeꢀpacked metal NP, only 35% of total atoms were exposed to the surface, as the
effective reaction medium.8 Therefore, to make full use of nobleꢀmetal element or reduce the
amount of nobleꢀmetal atoms without sacrificing the catalytic activity, many effective
methods have been reported. First, monatomic catalysts received significant attention. The
monatomic catalyst not only has a very low metal loading, but also significantly improves the
utilization efficiency of metal atoms. Zhang et al. first prepared monatomic Pt/FeOx catalyst
in 2011, exhibiting very high CO selective oxidation catalytic activity and stability.12
Recently, Zheng et al. reported a high loading (1.5 wt%) single atom Pd catalyst by a
photochemical method, exhibiting high activity and stability for the hydrogenation of
compounds containing C=C and C=O.11 However, when the metal particles reduced to the
monatomic level, the specific surface area increases sharply, resulting in a sharp increase in
the free energy of the metal surface. The large cluster formation during the preparation can
lead to catalyst deactivation. Moreover, the fabrication process of the monatomic catalysts is
also highly expensive, and the preparation of monatomic catalysts still faces many challenges
and is unsuitable for largeꢀscale application. Second, designing noble and nonꢀnoble metals
alloy catalysts is also an effective method to reduce the usage of noble metals. Alloy catalysts
contributing to metal interaction and noble metal shell lattice distortion showed synergistic
effects. In addition, nonꢀprecious metal also plays a vital role in controlling the catalytic
activity, selectivity, and stability. Li et al. reported oneꢀpot synthesis of PdNi alloy NPs
catalyst, exhibiting high activity and stability in catalyzing Suzuki–Miyaura coupling
reaction.13 However, the alloy catalysts still have small specific surface area and poor thermal
stability. Third, the core–shell structure nanocatalyst has received increasing attention, as the
inner noble atoms can be replaced by some other nonꢀprecious atoms, and this not only
improves the utilization of precious metal elements, but also newly introduced heteroatoms
play an important role to the nanocatalyst. Sun et al. prepared a 10ꢀnm Ni/Pd core–shell NPs
supported on graphene, exhibiting high catalytic activity for the Suzuki–Miyaura crossꢀ
coupling reaction.8 Although this method effectively improved the utilization of Pd atoms, the
relatively cumbersome preparation process hindered the largeꢀscale application. Therefore,
2

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designing simple and costꢀeffective method to prepare noble metal NPs based catalysts with a
maximum utilization of noble metals is a very meaningful and challenging task.
Nꢀdoped carbon materials are considered as potential support materials for heterogeneous
catalysis.14ꢀ16 Metal NPs deposited on Nꢀdoped carbon materials can be easily obtained by
pyrolyzing the mixture of inorganic metal salts and most common Nꢀcontaining organic
compounds at high temperatures (800–1000°C).17, 18 However, metal NPs generally undergo
agglomeration and thus cannot be well dispersed during pyrolysis. Metalꢀorganic frameworks
(MOFs) owing to its superb structure tenability surface also can be used as ideal sacrificial
templates for generating metal NPs deposited on Nꢀdoped carbon materials; however,
expensive organic ligands limit their largeꢀscale application. Hence, developing an
inexpensive, highly dispersed metal NPs, Nꢀdoped material is a very significant task. In
general, Pd NPs modified Nꢀdoped nanocatalysts can be used in various chemical reactions.
For example, the catalytic hydrogen generation from formic acid (FA)19 and the
hydrogenation of unsaturated compounds.20 FA as a nonꢀtoxic liquid at room temperature
with characteristic properties such as high energy density, nontoxic, and excellent stability
has received tremendous attention for H2 storage and generation. It can be easily obtained
from biomass processing in nature and produces CO2 as the exclusive product.21ꢀ23 Therefore,
using FA as a reductant is both sustainable and a very environmentally friendly raw material
with only CO2 as the byꢀproduct during the reaction towards sustainable green chemistry.
Besides, aromatic amines, usually obtained by reducing nitro compounds, are very important
intermediates in the production of various fine chemicals.5, 24, 25 Using FA as the hydrogen
source, nitroarenes can be hydrogenated to aromatic amines by a transfer hydrogenation
method.
Keeping the above considerations in mind, in this study, a highly dispersed Ni NPs
modified Nꢀdoped mesoporous carbon material (Ni/mCN) was prepared by facile pyrolysis of
a mixture of polyacrylonitrile (PAN), melamine, and Ni(NO3)2·6H2O at high temperature.
The obtained Ni/mCN nanocatalyst was treated with a solution of Pd2+, and the metallic Ni
NPs spontaneously reduced Pd2+ to metallic Pd, depositing the metallic Pd atoms on the
surface of Ni NPs. Thus, PdNi NPs modified Nꢀdoped mesoporous carbon materials
(PdNi/mCN) were prepared with maximum exposed Pd active sites. Comparing with other
catalysts, PdNi/mCN nanocatalyst exhibits excellent physicochemical properties with fluffy,
mesoporous, large special surface area, ideal surface composition and chemical state.
PdNi/mCN nanocatalyst showed excellent catalytic activity in the transfer hydrogenation of
nitroarenes to corresponding aromatic anilines, attributed to the highly dispersed PdNi NPs
modified Nꢀdoped mesoporous carbon and significantly exposed active site with Pd atoms on
the catalyst surface. Using aqueous solution as the reaction solvent further reflects the
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sustainability and green chemistry of the catalytic system. In addition, the PdNi/mCN
nanocatalyst not only can be easily recycled at least ten times, while maintaining its high
catalytic activity and stability, but also can be easily separated from the solution using an
external magnet because of the magnetic property of metal Ni. Therefore, the research work
presented here provides an effective method for the fabrication of noble metal based Nꢀdoped
carbon catalysts with maximum utilization of noble metals for further industrial applications.
Experimental
Materials
The raw materials including PAN, Ni(NO3)2·6H2O, melamine, FA and other reagents used
in all the experiments were purchased from Sinopharm Chemical Reagent Co., Ltd. (China).
All other reagents were used as received.
Preparation of Νi/mCN
Ni/mCN was prepared by sufficiently pyrolyzing a mixture of PAN, melamine, and
Ni(NO3)2·6H2O at a high temperature of 900 °C. The specific preparation method used is
described as follows: First, 2.12 g PAN and 650 mg Ni(NO3)2·6H2O were added to 50 mL
DMF at 100 °C and vigorously stirred for 30 min until completely dissolved. To that clear
solution, 30 g melamine was gradually added and the resulting reaction mixture was further
stirred vigorously for another 1 h to obtain a thick slurry mixture. The obtained thick slurry
was evaporated and dried overnight in an oven at 120 °C and ground into powder. Finally, the
obtained powder was pyrolyzed at 900 °C for 1 h by a programmed heating protocol in
nitrogen atmosphere at a rate of 5 °C/min in a tubular furnace, affording Ni/mCN.
Preparation of PdNi/mCN
The PdNi/mCN was prepared by spontaneously reducing Pd2+ to metallic Pd by Ni NPs.
500 mg Ni/mCN was reduced at 300 °C for 2 h in a nitrogen/hydrogen (VN :VH = 9:1)
2
2
atmosphere in a tubular furnace. The reduced powder was then rapidly added to 50 mL of
water solution to prevent excessive oxidation of air. Then, 2 mL Pd(AcO)2 solution
(containing 10 mg Pd2+) was added to the above system and refluxed at 65 °C to completely
reduce the Pd2+ to metallic Pd by Ni NPs, covering the surface of the Ni NPs. The solid
powder was separated using a magnet and dried in a vacuum oven, successfully affording
PdNi/mCN nanocatalyst.
The detailed preparation process of mCN, Pd/mCN, Ni/AC, and PdNi/AC nanocatalysts are
provided in the Supporting Information.
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Catalysis Test
Catalysis transfer hydrogenation of nitrobenzene was carried out under atmospheric
pressure at room temperature. In a typical reaction, 0.5 mmol of nitrobenzene (61.5 mg), 10
mg of the PdNi/mCN nanocatalyst, and 5 mL of solvent were added to a 10 mL roundꢀbottom
flask. Then, 2 mmol of FA as the hydrogen donor was added to the above reaction mixture. In
this reaction, different volume ratios of water and ethanol were used. Samples were taken
every 10 min through an injection syringe and filtered; the conversions of the reactions were
detected by gas chromatography–mass spectrometry (GCꢀMS, Agilent 5977E). The
nanocatalyst was separated from the reaction mixture using an external magnet.
In order to further illustrate the general applicability of the prepared nanocatalyst, the
transfer hydrogenation of other aromatic nitroꢀcompounds was also carried out under the
same condition as mentioned above.
Characterization
In this study, nanaocatalysts were prepared and characterized in detail to understand their
physical and chemical properties. The crystalline phase structures of the asꢀprepared catalysts
were investigated by Powder Xꢀray diffraction (XRD, Rigaku D/maxꢀ2400). Transmission
electron microscopy (TEM, FEI Tecnai G2 F30, 200kV) and scanning electron microscopy
(SEM, HITACHI Sꢀ4800) were used to observe the morphology of the nanocatalysts. The
surface electronic states of the catalyst were analyzed by Xꢀray photoelectron spectroscopy
(XPS, PerkinꢀElmer PHIꢀ5702). Brunauer–Emmett–Teller (BET, Micromeritics ASAP 2010)
analysis was used to investigate the pore size distributions, surface areas, and pore volumes of
nanocatalysts. Raman spectroscopy (Jobin Yvon LabRam HR) was used to record Raman
spectra. The Ni and Pd contents of the asꢀprepared nanocatalysts were evaluated by
inductively coupled plasmaꢀoptical emission spectrometry (ICPꢀOES, Agilent 5100 SVDV).
Vibrating sample magnetometry (VSM, MicroSense) was utilized at room temperature by
varying the applied magnetic field from −20 kOe to 20 kOe. The reaction conversion was
determined by GC–MS (Agilent 5977E).
Results and Discussion
In this study, PAN, melamine, and Ni(NO3)2·6H2O dissolved DMF mixture could form a
viscous sample. The preparation temperature of the precursor material Ni/mCN was firstly
investigated. From the TEM characterization, it can be clearly seen that, when the pyrolysis
temperature is low (<800 oC), the viscous sample can not be completely pyrolyzed, and the
o
Ni2+ can not be fully transferred to metallic Ni NPs. When pyrolyzed at 900 C, metallic Ni
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NPs and the porous mCN material are formed (Fig. S1). The PdNi/mCN nanocatalyst was
successfully prepared via spontaneously reducing Pd2+ by Ni NPs. The PdNi NPs with high
dispersion on the Nꢀdoped mesoporous carbon material (PdNi/mCN) and Pd atoms were
successfully deposited on the surface of Ni NPs for transfer hydrogenation of nitroarenes
using FA as the reductant under ambient conditions (Scheme 1). We also tried to prepare
Fe/mCN and Co/mCN to fabricate PdFe/mCN and PdCo/mCN nanocatalysts, respectively.
However, Fe and Co elements mainly exist in the form of oxides as shown in the XPS results
(Fig. S2). To further understand various physical and chemical properties of the nanocatalysts,
a series of characterization was carried out.
Scheme 1. The preparation procedure and the application of PdNi/mCN nanocatalyst.
The morphologies and microstructures of the prepared samples were characterized by SEM
and TEM. Figs. 1a–c show the SEM images of mCN, Ni/mCN, and PdNi/mCN. The SEM
images clearly show that the prepared samples have an incompact structure. The structures
did not obviously deformed when modified by Ni or PdNi NPs to mCN. Figs. 1d–f show the
TEM images of mCN, Ni/mCN, and PdNi/mCN, further illustrating the fluffy and
mesoporous morphology of the prepared samples. In Fig. 1e the Ni NPs were highly
dispersed on the mCN surface. The previous theoretical and experimental studies suggest that
defective CꢀN sites and N interstitials can be used as active centers for the nucleation and
growth of metal NPs.26, 27 Fig. 1e shows highly dispersed Ni NPs with uniform size. The TEM
image of PdNi/mCN nanocatalyst is shown in Fig. 1f, also exhibiting highly dispersed PdNi
NPs with a mean size of 3.5±0.5 nm as listed in Fig. 1f inset by randomly counting 300
particles. In Fig. 1g, a partial enlargement clearly demonstrates the high dispersion and
uniform size distribution of PdNi NPs. The unique structure of mCN can enhance the
accessibility of the active sites of PdNi NPs in the catalytic reaction process. The HRTEM
image (Fig. 1g, inset) further reveals that these Pd atoms were deposited on the surface of Ni
NPs also exhibiting two characteristic lattice distances of 0.204 and 0.34 nm, corresponding
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to the crystal plane of metallic Ni(111) and Pd(111), respectively.28 The element mappings
clearly indicate that C, N, Ni, and Pd atoms are uniformly distributed on the surface of mCN
(Fig. 1h). By carefully analyzing the distribution of Ni and Pd, it can be clearly seen that Ni
and Pd elements well fitted together, also explaining how Pd was successfully deposited on
the surface of Ni NPs. Thus, we can confirm that PdNi/mCN nanocatalyst with N element
uniformly doped into the carbon materials and PdNi NPs with Pd atoms on its surface were
successfully prepared.
Fig 1. The SEM images of (a) mCN, (b) Ni/mCN and (c) PdNi/mCN. The TEM images of (d) mCN, (e)
Ni/mCN and (f) PdNi/mCN (inset: the PdNi particleꢀsize distribution histogram). (g) The partial
enlargement TEM image of PdNi/mCN nanocatalyst (inset: the HRTEM of PdNi NPs), (h) the element
mapping images of PdNi/mCN nanocatalyst.
Nitrogen adsorption–desorption isotherms of the prepared nanocatalysts were also
measured. The prepared nanocatalysts exhibited type IV isotherms (Fig. 2a) with a mediumꢀ
pressure and the highꢀpressure region hysteresis loops, explaining that the nanocatalysts
mainly contain mesoporous structures together with a little microporous structures.29 The
large number of mesoporous structure can provide more surface area to enhance the
accessibility of the reaction substrates to the PdNi NPs. The texture parameters such as
surface area, average pore size, and pore volume of the nanocatalysts, mCN, Ni/mN, and
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PdNi/mCN nanocatalysts are listed in Table 1, exhibiting the large surface area of 93.9, 71.4,
and 72.6 m2g−1, respectively. The surface area, average pore size, and pore volume of mCN
are larger than those of Ni/mCN and PdNi/mCN, probably because of the Ni and PdNi NPs
inside the mCN, occupying a certain space. Moreover, the elemental contents of the prepared
nanocatalysts are also listed in Table 1 according to the element analysis and ICPꢀOES test
results. The N, Ni, and Pd loading of PdNi/mCN nanocatalyst are 5.5%, 10.7%, and 1.8%,
respectively. As expected, the content of Ni obvious decreases from Ni/mCN to PdNi/mCN,
and it can be easily explained that Pd2+ oxidized the surface of Ni NPs to from PdNi NPs,
further explaining that the metallic Pd was generated by spontaneously reducing Pd2+ by
metallic Ni NPs. In Fig. 2b, the poreꢀwidth distributions of the prepared samples did not
obvious change, demonstrating that the introduction of nanoparticles does not significantly
damage the structure of the Nꢀdoped carbon materials.
Fig.2. (a) Nitrogen adsorption/desorption isotherms of mCN, Ni/mCN and PdNi/mCN and (b)
corresponding poreꢀwidth distribution curves.
Table 1. Texture parameters and the element contents of the prepared nanocatalysts.
Surface
area
Average
pore size
(nm)
Pore
volume
(cm3 g− 1
N
Ni
Pd
Samples
(wt%) (wt%) (wt%)
(m2 g−1)
)
mCN
93.9
71.4
72.6
14.8
9.7
0.38
0.20
0.18
19.61
6.47
5.50
ꢀ
ꢀ
ꢀ
Ni/mCN
PdNi/mCN
11.3
10.7
9.5
1.8
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Fig. 3. (a) The wideꢀrange XPS spectra (mCN, Ni/mCN, PdNi/mCN), (b) Ni 2p spectra (Ni/mCN,
PdNi/mCN), (c) N 1s spectra (mCN, Ni/mCN, PdNi/mCN) and (d) Pd 3d spectrum of PdNi/mCN.
XPS measurements were carried out to further investigate the surface composition and
chemical state of the prepared catalysts. In Fig. 3a the wideꢀrange XPS spectrum clearly
exhibits the presence of C, N, Ni, and Pd elements in different nanocatalysts. In addition, the
energy dispersive spectrum (Fig. S3) also exhibits the same compositions of the PdNi/mCN
nanocatalyst, corresponding to the results in Fig. 3a. Using the C 1s peak at 284.6 eV as the
reference peak (Fig. S4), the Ni 2p spectra shown in Fig. 3b clearly confirm that the Ni
element mainly exists in the form of metallic Ni in Ni/mCN sample. When spontaneous
reduction occurs between Ni/mCN and Pd2+, the intensities of Ni 2p peaks decrease obviously,
mainly due to the metallic Pd atoms covered on the surface of metallic Ni NPs. Fig. 3c
exhibits the highꢀresolution XPS spectra of N 1s for mCN, Ni/mCN, and PdNi/mCN. The N
1s spectrum can be divided into four different fitted peaks, corresponding to different N
species Pyriclinic N (398.6 eV), pyrrolic N (399.8 eV), graphitic N (401.1 eV), and oxidation
N (402.5 eV).30ꢀ32 The percentages of N element in different nanaocatalysts are listed in Table
2 based on the peak areas of the N 1s highꢀresolution XPS spectra fitted peaks. The content of
Pyridinic N decreased significantly from 43.6% to 5.1% when Ni(NO3)2·6H2O was added to
the mixture of PAN and melamine for pyrolysis. Hence, Pyridinic N plays a crucial role in the
process of forming Ni NPs. It further explained that Pyridinic N can be used as the active
centers for the nucleation and growth of metal NPs. Fig. 3d shows the highꢀresolution
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spectrum of Pd element, exhibiting the characteristic peaks at 335.5 and 340.8 eV
corresponding to the Pd 3d3/2 and Pd 3d5/2 transitions, respectively.2 The Pd element mainly
exists as Pd0. A small amount of Pd2+ may be because the naked metal Pd atoms on the
surface of nanocatalyst can be easily oxidized to form Pd oxide under the ambient conditions
or coordinate to N atoms in the supporting material mCN.
Table 2. Different surface valence states of N and Pd in Ni@Pd/mCN nanocatalyst from XPS.
Pyridinic N
398.6 eV
43.6%
Pyrrolic N
399.8 eV
25.9%
Graphitic N
401.2 eV
18.7%
Nꢀoxide
402.5 eV
9.1%
Samples
Pd0
Pd2+
mCN
ꢀ
ꢀ
ꢀ
ꢀ
Ni/mCN
PdNi/mCN
5.1%
54.6%
19.9%
20.4%
11.6%
5.2%
66.6%
16.6%
88.8% 11.2%
Fig. 4a shows the XRD analysis of the prepared nanocatalysts. The XRD curves of mCN,
Ni/mCN, and PdNi/mCN nanocatalyst show a broad peak at 26.4°, attributing to the
diffraction peak of graphite carbon (JCPDS No.25ꢀ284). In addition, the characteristic peaks
at 44.6, 52.0, and 76.5° corresponded to the (111), (200), and (220) planes, respectively, of the
faceꢀcentered cubic (fcc) structure of Ni/mCN and PdNi/mCN (JCPDS: 70ꢀ989).
Corresponding to metallic Pd, there is only one less obvious faceꢀcentered cubic (fcc)
structure of (111) in the XRD pattern of PdNi/mCN nanocatalyst. Because Pd had a lower
surface tension and a larger atomic radius than Ni, it tends to be expelled from the Ni matrix,
34
leading to a strong Pd segregation.33,
In contrast, this may be because of too low
concentration of metallic Pd or inhomogeneous distribution of Pd phase and also suggests that
small crystals of the metal Pd NPs (<5 nm)35 are hard to detected by XRD.36 This can also
confirm the formation of the PdNi NPs with Pd atoms on their surface.
Raman spectroscopy is also an effective technique to understand the incorporation of
heteroatoms in carbon materials.37 Fig. 4b shows the Raman spectra of the mCN, Ni/mCN,
and PdNi/mCN nanocatalysts. Two major characteristic bonds at 1580 cmꢀ1 (G) and 1360 cmꢀ1
(D) were assigned to the graphene sheet and the graphitic defect state of carbon,
respectively.15, 38, 39 The intensity ratio between the D and G peaks (ID/IG) usually considered
to be a carbon material defect level. The mCN, Ni/mCN, and PdNi/mCN show the ID/IG of
1.10, 1.09, and 1.12, respectively. These results indicate that the carbon materials have certain
defects caused by nitrogen atoms, thus indicating that nitrogen atoms are successfully doped
into the carbon material, forming C–N structure.
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Fig.4. (a) XRD patterns of mCN, Ni/mCN and PdNi/mCN nanocatalyst, and (b) Raman spectra of
mCN, Ni/mCN and PdNi/mCN nanocatalyst.
The magnetic property is a very important factor to evaluate the recoverability of a
heterogeneous catalyst. Fig. 5 exhibits the magnetization curves of Ni/mCN and PdNi/mCN
nanocatalysts at room temperature. The VSM curves show that Ni/mCN and PdNi/mCN have
a saturation magnetization of approximately 19.08 and 17.89 emu g−1, respectively,
contributing magnetic characteristic to Ni NPs. This magnetic property is essential for catalyst
40
to be recycled from the reaction mixture solution.6,
It is apparent that the magnetic
properties of the catalyst slightly deteriorated when the PdNi/mCN was prepared by
spontaneously reducing Pd2+ by Ni NPs.
Fig. 5. Roomꢀtemperature magnetization curves of Ni/mCN and PdNi/mCN nanocatalysts (Inset is the
magnetic separate process of nanocatalysts).
In order to illustrate the application and catalytic activity of the prepared PdNi/mCN
nanocatalyst, transfer hydrogenation of nitrobenzene was carried out as the model reaction
using HCOOH as the reductant at ambient conditions. The results of the transfer
hydrogenation of nitrobenzene in different solvents are summarized in Table 3. To compare
the activity of the nanocatalysts in different solvents, the Turnover frequency (TOF) was
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introduced. The results indicate that different volume ratios of water and ethanol in the
reaction result in a significant change. Using ethanol as the solvent only produced 55%
aniline in 180 min. With increasing volume of water, the yield of aniline continued to increase
(Entries 1–3), and the TOF increases from 54.8 hꢀ1 to 591.2 hꢀ1. Changing the solvent greatly
increases the activity of the PdNi/mCN nanocatalyst. To a certain extent, the catalytic activity
significantly reduced with increasing volume of water (Entries 3–7). The TOF decreased
from 591.2 hꢀ1 to 26 hꢀ1. Therefore, we believe that water plays a vital role in the process of
transfer hydrogenation of nitrobenzene, promoting the ionization of FA to form H+. The
catalyst has the highest catalytic activity when the volume of ethanol/water is 3:2 (Entry 3,
E:W=3:2). To further clearly illustrate the effect of solvent on the catalyst activity, the
conversion curves at different times are also shown in Fig. S5.
Table 3. Transfer hydrogenation of nitrobenzene over different volume ratio of ethanol and water
(E:W) as solvent.a
TOFb
Reaction
time (min)
180
Conversion
(%)
Entry
Solvent
(h-1)
54.8
Ethanol
E:W=4:1
E:W=3:2
E:W=2.5:2.5
E:W=2:3
E:W=1:4
Water
55.6
95.5
100
100
100
81.7
26.4
1
2
3
4
5
6
7
180
30
60
100
180
180
94.1
591.2
295.6
177.4
80.5
26.0
aReaction conditions: 0.5 mmol substrate, 10 mg PdNi/mCN nanocatalyst, 2 mmol HCOOH, and 5 mL
solvent at ambient conditions.
ꢂꢃꢄꢅꢆ ꢃꢇ ꢈꢃꢉꢊꢅꢋꢌꢅꢍ ꢆꢎꢏꢆꢌꢋꢐꢌꢅ
b
ꢀ
ꢁ
Turnover frequency TOF = ꢂꢃꢄꢅꢆ ꢃꢇ ꢑꢍ× ꢋꢅꢐꢈꢌꢒꢃꢉ ꢌꢒꢂꢅ ꢓ
.
ꢁ
ꢀ
To further confirm the advantage of PdNi/mCN, a series of nanocatalysts such as Pd/AC,
PdNi/AC, Pd/mCN, Ni/mCN, and mCN were also prepared for comparison (Table 4). Pd/AC
(Figs. S6a) and PdNi/AC (Figs. S6c) both have larger particle size when using the AC as the
supporting material as compared with PdNi/mCN nanocatalyst (PdNi average size: 3.5 nm).
This should attribute the nitrogen atoms that highly dispersed in the mCN can act as the
central active sites for the nucleation and growth of metal NPs, thus PdNi NPs in PdNi/mCN
are more dispersed than PdNi/AC and Pd/AC. Therefore, Entry 1 shows that the catalytic
activity of PdNi/mCN is significantly higher than other nanocatalysts (TOF value: 591.2 hꢀ1).
Entries 2 and 3 clearly show similar TOF values of 199.6 and 199.5 hꢀ1 for PdNi/AC and
commercial Pd/AC, respectively, with the same reaction time. Entry 4 shows that Pd/mCN
activity is also significantly stronger than those of Pd/AC and PdNi/AC, and weaker than
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PdNi/mCN. Because of the presence of N atoms in mCN, the Pd NPs were highly dispersed
than Pd/AC (the TEM image shown in Fig. S6e). In PdNi/mCN nanocatalyst, since Pd atoms
were wrapped on the surface of Ni NPs, more active sites were exposed, allowing maximum
utilization of Pd atoms. Therefore, PdNi/mCN nanocatalyst with a Pd content of 1.8% has
higher catalytic activity than Pd/mCN with a Pd content of 2%. In Entries 5–9, the transfer
hydrogenation of nitrobenzene was carried out under the same conditions using Ni/mCN,
Ni/AC, mCN, AC, and without catalyst. Because there is no active elemental Pd, only a small
yield of aniline was observed. Therefore, Ni only exhibited magnetic characteristic and did
not act as an active catalyst ingredient.
Table 4. Transfer hydrogenation of nitrobenzene over different nanocatalyststs.a
TOFb
Reaction
time (min)
30
Conversion
(%)
Entry
1
Catalyst
(h-1)
PdNi/mCN
(1.8%)
PdNi/AC
(2%)
Pd/AC
(5%)
100
591.2
80
80
50
99.6
99.3
99.8
199.6
199.5
354.7
2
3c
4
Pd/mCN
(2%)
Ni/mCN
Ni/AC
mCN
180
180
180
180
180
2.1
1.8
1.5
1.4
1.4
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
5
6
7
8
9
AC
ꢀ
a
Reaction conditions: 0.5 mmol substrate, 10 mg nanocatalyst, 2 mmol HCOOH, and 5 mL solvent
ꢂꢃꢄꢅꢆ ꢃꢇ ꢈꢃꢉꢊꢅꢋꢌꢅꢍ ꢆꢎꢏꢆꢌꢋꢐꢌꢅ
(E:W=3:2) at ambient conditions. b Turnover frequency TOF = ꢂꢃꢄꢅꢆ ꢃꢇ ꢑꢍ× ꢋꢅꢐꢈꢌꢒꢃꢉ ꢌꢒꢂꢅ ꢓ
.
ꢁ
ꢀ
ꢁ
ꢀ
c
4 mg commercial Pd/C (5%).
The possible reaction mechanism of the transfer hydrogenation of nitrobenzene with FA as
the hydrogen source using the PdNi/mCN is shown in Scheme 2. Because PdNi/mCN has a
mesoporous microporous structure, nitrobenzene and HCOOH were first adsorbed on the
surface of the mesoporous microporous structure and attached on the surface of PdNi NPs.
Then, HCOOH dissociated into two protons, CO2 molecules, and two electrons on the surface
of Pd shells in the presence of water. The two protons and electrons were captured by
nitrobenzene to form nitrosobenzene and CO2 was released to solvent. The nitrosobenzene
might quickly convert to phenylhydroxylamine, which can be easily combined with hydrogen
ions and release water to form aniline; in contrast, phenylhydroxylamine can also react with
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nitrosobenzene to form azobenzene. Because azobenzene can be rapidly converted to aniline
under acidic conditions, only trace amounts of azobenzene can be detected in the course of
the reaction (the azobenzene was detected by GC–MS).41ꢀ43
Scheme 2. The possible reaction mechanism for transfer hydrogenation of nitrobenzene to aniline over
PdNi/mCN.
To further demonstrate the extensive applicability of the PdNi/mCN nanocatalyst, the
transfer hydrogenation of various substituted nitroarenes were examined using this
nanocatalyst. The catalytic results are summarized in Table 5. In Table 5, Entries 1–6,
halogenated nitrobenzenes were first examined, indicating that a variety of halogenated
nitrobenzenes have high conversion and excellent selectivity to the corresponding
halogenated anilines. Pd is well known to have a very strong catalytic activity for the
24
hydrodehalogenation of halogenated aromatic compound.4,
But using PdNi/mCN
nanocatalyst and HCOOH as the reductant, except 4ꢀchloronitrobenzene (Table 5, entry 2)
with only a selectivity of 84.4%, other halogenated nitrobenzenes exhibited >95% selectivity
to the corresponding halogenated anilines. In Table 5, Entries 7–15, in the transfer
hydrogenation of nitrophenol (Entries 7–9), nitrotoluene (Entries 10–12), and nitroaniline
(Entries 13–15), the PdNi/mCN nanocatalyst also exhibited outstanding catalytic activity.
The transfer hydrogenation of 3ꢀmethylꢀ2ꢀnitroaniline (Entry 16) has a conversion of 99.5%;
however, that of 4ꢀnitrobenzaldehyde (Entry 17) only has 96.5% yield of 4ꢀ
aminobenzaldehyde, because of the excessive FA, partially hydrogenating the C=O bond
under the action of Pd, thus slightly decreasing the yield of 4ꢀaminobenzaldehyde. The above
results demonstrate the extensive applicability of the PdNi/mCN nanocatalyst for the transfer
hydrogenation of the substituted nitrobenzene.
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Table 5. Transfer hydrogenation of nitroarenes to their amines with PdNi/mCN nanocatalyst at
optimal conditions.a
Reaction Conversion Selectivityb
Entry
Substrate
Product
(min)
(%)
(%)
1
2
40
100
95.0
80
97.8
84.4
3
4
80
80
99.2
100
95.5
100
5
60
99.4
97.8
6
7
8
80
60
60
99.3
99.8
99.0
96.7
100
100
9
60
60
100
100
100
10
96.8
11
80
99.4
100
12
13
14
60
40
60
99.5
100
100
100
100
98.6
15
60
99.1
100
16
17
60
50
99.5
100
100
96.5
a
Reaction conditions: 0.5 mmol substrate, 10 mg PdNi/mCN nanocatalyst, 2 mmol HCOOH, and 5 mL
solvent (E:W=3:2) at ambient conditions.
b
The selectivity of correspond anilines were detected by GCꢀMS.
Finally, the reusability of the PdNi/mCN nanocatalyst was also investigated for the transfer
hydrogenation of nitrobenzene to aniline (Fig. 6), indicating that after ten cycles, the yield of
the aniline can still reach to 97%. Because of the presence of Ni NPs, the catalyst can be
easily separated from the solution using a magnet. The recycled nanocatalyst was washed
three times with ethanol and dried in a vacuum oven for 12 h for the next cycle. The ICPꢀOES
analysis of the recycled catalyst shows only a slight decrease in the Pd content from its
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original value of 1.8 wt% to 1.6 wt%. This decrease should be because of the acidic reaction
condition, leading to the slight leaching of the metallic Pd deposited on the surface of the Ni
NPs. Besides, the binding energy of metal species to the Nꢀdoped carbon support is high and
can improve the stability of the metal NPs.44 The used PdNi/mCN nanocatalyst was also
characterized. The TEM (Fig. S7a), VSM (Fig. S7b), BET (Fig. S7c), and XRD (Fig. S7d)
analyses show that the morphology, magnetic performance, porous structure, and crystal
structure of the nanocatalyst did not show any obvious change. Therefore, through ten cycles
of the reaction, PdNi/mCN nanocatalyst still shows a high catalytic activity and stability.
Fig. 6. Recycle of the PdNi/mCN nanocatalyst (Reaction conditions: 0.5 mmol substrate, 10 mg
PdNi/mCN nanocatalyst, 2 mmol HCOOH, and 5 mL solvent (E:W=3:2) at ambient conditions.
Conclusions
In this study, magnetically separable and highly dispersed PdNi NPs modified Nꢀdoped
mesoporous carbon nanocatalyst was successfully prepared by a simple method. The obtained
PdNi NPs with Pd atoms on the NPs surface not only increase the utilization of Pd, but also
can be easily separated from the solution using an external magnet because of the magnetic
property of metallic Ni. In the transfer hydrogenation reaction of nitroarenes, the PdNi/mCN
nanocatalyst exhibited excellent catalytic activity. The Nꢀdoped mesoporous carbon materials
here can provide a large surface area to adsorb the reductant and substrate and enhance the
accessibility of the active sites PdNi NPs in the catalytic reaction process. Using aqueous
solution as the solvent and FA as the reductant reflect the catalytic system of sustainable
green chemistry concept. Therefore, the work reported here would provide an effective
strategy for preparing nobleꢀmetal modified porous Nꢀdoped carbon supported catalysts with
the maximum utilization of noble metals for sustainable green chemistry.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank for the financial support from the Fundamental Research Funds for the
Central Universities (lzujbkyꢀ2017ꢀ112).
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Graphical Abstract