A facile method for the synthesis of graphene-like 2D metal oxides and their excellent catalytic application in the hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c8ta02338g

Graphene-like two-dimensional (2D) metal oxides in the form of nanosheets (NSs) often exhibit superior performance in many applications in comparison with their bulk counterparts, owing to their unique electronic and physicochemical properties arising from their layered 2D structure; however, it is still a great challenge to produce 2D metal oxides in a facile way, especially for non-layered metal oxides. In the present work, a modified evaporation induced self-assembly (EISA) method was developed to synthesize graphene-like CeO₂ nanosheets (NS-CeO₂). Moreover, the EISA method is proved to be rather universal and a series of other single and mixed metal oxides, including TiO₂, ZrO₂, CoO_x, NiO, Al₂O₃, CuO, CuO-ZrO₂, MnO_x-Al₂O₃, FeO_x-CoO_x, Cr₂O₃-ZnO, CeO₂-ZrO₂, CeO₂-CuO, CeO₂-CoO_x, CeO₂-La₂O₃, CeO₂-FeO_x, CeO₂-MnO_x, CeO₂-ZnO, CeO₂-Al₂O₃, CeO₂-NiO, CeO₂-Cr₂O₃, CeO₂-TiO₂, MnO_x-CeO₂-ZrO₂, and CuO-CeO₂-ZrO₂, can be successfully prepared in the form of nanosheets with tunable pore structure and sheet thickness by using this method. The catalytic application of the graphene-like metal oxides was considered and the results illustrate that the CeO₂ nanosheets decorated with Pd clusters (Pd/NS-CeO₂) exhibit excellent catalytic performance in the hydrogenation of nitroarenes under mild conditions; over them, nitrobenzene can be completely converted to aniline even at room temperature in a short time. A series of characterization results combined with DFT calculations demonstrate that the CeO₂ nanosheets are enriched with surface defects and Ce³⁺ species, which can promote the adsorption and activation of nitroarenes. The present work brings forward a facile and effective approach to synthesize graphene-like metal oxides with superior catalytic performance; besides, the results obtained in this work are also rather helpful to clarify the relationship between the 2D structure and performance of the Pd/NS-CeO₂ catalyst in the hydrogenation of nitroarenes.

Full text of DOI:10.1039/c8ta02338g

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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A facile method for the synthesis of grapheneꢀlike 2D metal oxides and
their excellent catalytic application in the hydrogenation of nitroarenes †
Lijun Lei,a,b Zhiwei Wu*,a Huan Liu,a,b Zhangfeng Qin,*,a Chengmeng Chen,c Li Luo,a,b
Guofu Wang,a Weibin Fan,a and Jianguo Wang*,a
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of
Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China
b University of the Chinese Academy of Sciences, Beijing 100049, PR China
c Key Laboratory of Carbon Materials, Institute of Coal Chemistry, the Chinese Academy of
Sciences, Taiyuan 030001, PR China
* Corresponding authors. Tel.: +86ꢀ351ꢀ4046092; Fax: +86ꢀ351ꢀ4041153. Eꢀmail address:
qzhf@sxicc.ac.cn (Z. Qin); iccjgw@sxicc.ac.cn (J. Wang); wuzhiwei@sxicc.ac.cn (Z. Wu)
† Electronic supplementary information (ESI) available: More experimental details and
results of catalyst characterization and reaction tests. See DOI: 10.1039/xxxxxxxxxx
Abstract: Grapheneꢀlike twoꢀdimensional (2D) metal oxides in the form of nanosheets (NS)
often exhibit superior performance in many applications in comparison with their bulk
counterparts, owing to the unique electronic and physicochemical properties arising from the
layered 2D structure; however, it is still a great challenge to produce 2D metal oxides in a
facile way, especially for the nonꢀlayered metal oxides. In the present work, a modified
evaporation induced selfꢀassembly (EISA) method was developed to synthesize the
grapheneꢀlike CeO2 nanosheets (NSꢀCeO2). Moreover, the EISA method is proved to be
rather universal and a series of other single and mixed metal oxides, including TiO2, ZrO2,
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CoOx, NiO, Al2O3, CuO, CuOꢀZrO2, MnOxꢀAl2O3, FeOxꢀCoOx, Cr2O3ꢀZnO, CeO2ꢀZrO2,
CeO2ꢀCuO, CeO2ꢀCoOx, CeO2ꢀLa2O3, CeO2ꢀFeOx, CeO2ꢀMnOx, CeO2ꢀZnO, CeO2ꢀAl2O3,
CeO2ꢀNiO, CeO2ꢀCr2O3, CeO2ꢀTiO2, MnOxꢀCeO2ꢀZrO2, and CuOꢀCeO2ꢀZrO2, can be
successfully prepared in nanosheets with tunable pore structure and sheet thickness by using
this method. The catalytic application of the grapheneꢀlike metal oxides was considered and
the results illustrate that the CeO2 nanosheets decorated with Pd clusters (Pd/NSꢀCeO2)
exhibit excellent catalytic performance in the hydrogenation of nitroarenes under mild
conditions; over it, nitrobenzene can be completely converted to aniline even at room
temperature in a short time. A series of characterization results combined with DFT
calculation demonstrate that the CeO2 nanosheets are enriched with surface defects and Ce3+
species, which can promote the adsorption and activation of nitroarenes. The present work
brings forward a facile and effective approach to get grapheneꢀlike metal oxides with
superior catalytic performance; besides, the results obtained in this work are also rather
helpful to clarify the relationship between the 2D structure and performance of Pd/NSꢀCeO2
catalyst in the hydrogenation of nitroarenes.
Keywords: Grapheneꢀlike 2D metal oxides; Evaporation induced selfꢀassembly;
Controllable synthesis; Nanosheets; Ceria; Palladium; Hydrogenation of nitroarenes.
1. Introduction
Owing to the unique properties and superior performance in various applications,
graphene as a twoꢀdimensional (2D) carbon material has attracted particular attention over
the past decades.1–4 The achievements made in the utilization of graphene have also inspired
great interest in the research of other 2D materials, including hexagonal boron nitride (hꢀBN),
graphite carbon nitride (C3N4), transition metal dichalcogenides (such as MoS2 and WS2),
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layered metal oxides, silicene, germanene, borophene, etc.5,6 In comparison with their bulk
counterparts, these grapheneꢀlike 2D materials with unique electronic, optical, mechanical
and chemical properties have shown enormous potential in various applications such as the
highꢀspeed electronic and optical devices, energy generation and storage, hybrid materials,
and catalysis.7–12
The methods to get ultrathin 2D materials can be generally divided into two categories,
viz., bottomꢀup and topꢀdown ones.9, 13 The topꢀdown methods, involving the liquid
exfoliation assisted by mechanical force, heat, oxidation, and selective etching, are capable
to prepare 2D materials from the parent bulk crystals with layered structures, such as
graphene, C3N4, MoS2, WS2, and hydrotalcite. As for the naturally nonꢀlayered materials, it
is rather difficult to get the grapheneꢀlike 2D materials via the topꢀdown route and then
various methods have been attempted to synthesize the 2D materials via the bottomꢀup
route. 14 – 19 For example, Xie, Kim, and coꢀworkeres fabricated several ultrafine 2D
nanoshteets of transition metal oxides (TMOs) by the hydrothermal and solvothermal
methods.17,18 With graphene oxide (GO) as the 2D template, as reported by Li, Peng,
Takenaka, and coꢀworkers, grapheneꢀlike TMOs nanosheets could be prepared by calcining
the metal precursors preciously deposited on the GO surface.15,16,19 Despite the advantages of
grapheneꢀlike 2D materials in various areas and the achievements recently made in the
preparation methods, it is still a great challenge to fabricate 2D metal oxides in a facile and
efficient way, especially for the naturally nonꢀlayered metal oxides.
Like graphene, the 2D metal oxides are also regarded as promising catalyst materials,
owing to their unique electronic and physiochemical properties, especially for its large
surface area, abundant surface defect sites, and strong interaction with the supported active
components. In this aspect, the hydrogenation of organic nitro compounds to amines aroused
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our great interest. As well known, amines are widely used in the production of
pharmaceuticals, pigments, agrochemicals, etc. and the hydrogenation of organic nitro
compounds is generally realized on supported metal catalysts.20–27 Many materials such as
active carbon,28– 31 carbon nanotubes,32–34 graphene,35, 36 MCMꢀ41 molecular sieves,37
siliconꢀnanowireꢀarray,38 and alumina39 have been used as the catalyst support. As the
active metal component was usually present in the reduced form during the hydrogenation
reactions, the reducible metal oxides were rarely used as a support for the metal catalysts.
However, more and more researchers nowadays have realized that the partially reducible
metal oxides as a support can greatly enhance the selectivity to a given product and/or the
stability for the hydrogenation reactions, because of the strong interaction between the metal
component and the reducible support.40–43 Besides, it was found that the nitro compounds
were easily adsorbed on a reducible support;44 especially, due to the unique surface
properties of CeO2 with the dynamically reversible Ce3+/Ce4+ redox pairs, Pd clusters
supported on the porous CeO2 nanorods exhibited high catalytic activity and selectivity in
the hydrogenation of nitroarenes.22 Taking into account the unique properties of 2D materials
and reducible metal oxides, there is an optimistic expectation toward the catalytic
performance of metal supported on reducible 2D metal oxides in the hydrogenation of
nitroarenes to amines.
In this work, a modified evaporation induced selfꢀassembly (EISA) method with
graphene oxide (GO) as template was developed for the synthesis of 2D metal oxide
nanosheets. A series of grapheneꢀlike 2D metal oxides, including single (CeO2, TiO2, ZrO2,
CoOx, NiO, Al2O3, and CuO), binary (CeO2ꢀZrO2, CeO2ꢀCuO, CeO2ꢀCoOx, CeO2ꢀLa2O3,
CeO2ꢀFeOx, CeO2ꢀMnOx, CeO2ꢀZnO, CeO2ꢀAl2O3, CeO2ꢀNiO, CeO2ꢀCr2O3, CeO2ꢀTiO2,
CuOꢀZrO2, Cr2O3ꢀZnO, FeOxꢀCoOx, MnOxꢀAl2O3), and ternary (MnOxꢀCeO2ꢀZrO2, and
CuOꢀCeO2ꢀZrO2) ones, were prepared by this general and facile method.
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As an example, especially, the uniform 2D CeO2 nanosheets (NSꢀCeO2) display a lateral
size of several micrometers and a thickness of only 3.7 nm; moreover, the porous CeO2
nanosheets have abundant surface Ce3+ species and oxygen vacancies. To explore the
potential of NSꢀCeO2 as a catalytic material, the catalytic performance of Pd clusters
supported on NSꢀCeO2 (Pd/NSꢀCeO2) in the hydrogenation of nitroarenes was investigated.
Surprisingly, for the hydrogenation of various nitroarenes, the Pd/NSꢀCeO2 catalyst exhibits
high activity, high selectivity to amines, and excellent stability upon repetitive utilization;
nitrobenzene can be completely converted to aniline at 25 °C in a short time. A series of
characterizations combined with DFT calculations were used to demonstrate the advance of
2D metal oxides as a catalyst support and to clarify the relationship between the 2D structure
and the catalytic performance of Pd/NSꢀCeO2 in the hydrogenation of nitroarenes.
2. Experimental and computational methods
2.1. Synthesis of 2D metal oxides
The graphene oxide (GO) used in this work was obtained by a modified Hummers’
method (ESI†);45 other reagents used in this work were purchased from Sinopharm
Chemical Reagent Co., Ltd., China, in the AR grade. The porous 2D nanosheets of various
metal oxides were synthesized by a modified process of evaporation induced selfꢀassembly
(EISA) involving two steps; that is, the metal nitrate precursors were first homogeneously
deposited on the surface of graphene oxide (GO) by the modified EISA method and GO
template was then removed through heat treatment to construct the 2D metal oxides.
Briefly, GO sheets were first suspended and homogeneously dispersed in ethanol under
ultrasonic irradiation for 1 h; subsequently, a diluted ethanol solution of nitrate precursors
(e.g. Ce(NO3)3·6H2O) was added slowly and the mixture was subjected to ultrasonic
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irradiation for another 1 h. After that, the mixture was stirred vigorously for 24 h and then
poured into multiple dishes; the system temperature was maintained at 25 °C in this step.
The dishes were placed in an electric oven to evaporate ethanol at 40 °C for 10 h, and 60 °C
for another 10 h; the residual matters were then dried at 80 °C in vacuum for 6 h. The
resulting darkꢀbrown films were then scratched from the dishes and ground into fine
powders in a mortar. The resulting dark brown powder was named as GOꢀM (e.g. GOꢀCe)
complex. In the next step, the calcination process was conducted in a muffle furnace under
static air to get the 2D metal oxides in nanosheets (NSꢀMOx, e.g. NSꢀCeO2); the furnace
temperature was programmed from room temperature to 450 °C at a ramp of 2 °C min–1 and
held at this temperature for 8 h; as an example, porous NSꢀCeO2 nanosheets in fluffy and
paleꢀyellow powder were then obtained. Depending on the temperatures for drying the
GOꢀM complex (Tdry, in °C) and for calcination to remove the GO template (Tcal, in °C), the
complex of metal oxides (MOx) assembled on GO and 2D MOx nanosheets after removing
GO template are denoted as GOꢀMꢀTdry (e.g. GOꢀCeꢀ80) and NSꢀMOxꢀTcal (e.g.
NSꢀCeO2ꢀ450), respectively. For comparison, bulk CeO2 (BꢀCeO2) and ZrO2 (BꢀZrO2) were
also prepared by the conventional precipitation method with ammonia as the precipitant.
NSꢀCeO2 supported Pd clusters (Pd/NSꢀCeO2) were prepared by a liquid reduction
method. Briefly, 0.5 g NSꢀCeO2ꢀ450 was first dispersed in 200 mL of water under ultrasonic
radiation for 0.5 h and a diluted aqueous solution of H2PdCl4 was then added slowly, stirring
vigorously for 24 h. After that, freshly prepared aqueous solution of NaBH4 (the molar ratio
of NaBH4 to Pd was 5) was carefully added to the reaction system and stirred overnight.
After filtration and rinsing with water and ethanol for three cycles, the Pd/NSꢀCeO2 catalyst
sample was obtained by further drying in vacuum at 60 °C for 8 h. For comparison, the
catalyst prepared by wet impregnation method, denoted as PdꢀIM/NSꢀCeO2, was reduced in
hydrogen at 200 °C for 2 h. The actual loadings of Pd in Pd/NSꢀCeO2 and PdꢀIM/NSꢀCeO2
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was 1.58 and 1.97 wt.%, respectively, as determined by the inductively coupled plasma
optical emission spectroscopy (ICPꢀOES) on a Thermo iCAP 6300 ICPꢀOES with a CID
detector.
2.2. Material characterization
Scanning electron microscopy (SEM) images were collected on a field emission
scanning electron microscope (FESEM, JSM 7001ꢀF, JEOL, Japan). Transmission electron
microscopy (TEM) observations were made on a field emission microscope (JEMꢀ2100F,
JEOL) equipped with an energy dispersive Xꢀray spectrometer (EDS) at an acceleration
voltage of 200 kV.
The powder Xꢀray diffraction (XRD) patterns were collected on a Bruker
D8ꢀAdvanceꢀA25 Xꢀray diffractometer with CuKα radiation source. The measurements were
made in the 2θ range from 10° to 80° with a scanning rate of 4 min−1.
The textural properties were determined by nitrogen adsorption at 77 K on a
physisorption analyzer (Tristar 3020, Micromeritics Instrument Co., USA); prior to the
measurement, the material samples were degassed at 200 °C and 6.7 Pa for 2 h. The specific
surface area was derived by using the Brunauer–Emmett–Teller (BET) method and the pore
size distribution was calculated from the adsorption isotherms by using the
Barrett–Joyner–Halenda (BJH) method.
Xꢀray photoelectron spectroscopy (XPS) was measured on a Thermo ESCALAB 250
instrument with an Al Kα monochromator Xꢀrays source (hv = 1486.6 eV); approximate 100
mg of the powder sample was compressed into a wafer for the measurement.
Raman spectra were recorded on a Labram HR800 Raman spectrometer equipped with a
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514 nm Ar+ laser (Horiba JobinꢀYvon).
Thermogravimetric analysis (TGA) was carried out on a Rigaku Thermo plus Evo TG
8120 analyzer; the sample cell was heated in air from room temperature to 900 °C with a
ramp of 10 °C min−1.
Temperature programmed reduction by hydrogen (H2ꢀTPR) was performed on an
automatic chemisorption analyzer (Micromeritics AutoChem II 2920) equipped with a
thermal conductivity detector (TCD) and a mass spectrometer; about 30 mg of the catalyst
sample was used in each measurement. To remove any surfaceꢀadsorbed impurities, the
catalyst sample was first pretreated in air (10 mL min−1) at 300 °C for 1 h, followed by
purging with argon (10 mL min−1) at the same temperature for 1 h, and then the sample was
cooled down to room temperature. After that, the flow of 10% H2 diluted in argon (H2–Ar
mixture, 10 mL min−1) was switched into the system and the sample was then heated up from
room temperature to 1000 °C at a ramp of 10 °C min−1. The quantity of hydrogen uptake
during the heating for reduction was recorded by the TCD, which was calibrated by
quantitative reduction of a given quantity of CuO to metallic copper.
Atomic force microscope (AFM) measurement was made on a scanning probe
microscope (Tip mode, 0.803 Hz, Veeco NanoScope IIIa Multimode, DI, USA); the probe
sample was prepared by precisely depositing the ethanolꢀdispersed NSꢀCeO2 on freshly
cleaved wafer surface.
The contents of elements (C, S) in the samples were determined by the elemental
microanalysis (EA, vario MICRO cube, Elementar).
Xꢀray absorption nearꢀedge spectroscopy (XANES) measurements were performed at the
beam line BL14W1 of Shanghai Synchrotron Radiation Facility, China. The beamline
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employed Si(311) monochromator for Pd Kꢀedge energy selection, with a resolution of ∆E/E
higher than 2×10−4. The Pd Kꢀedge XANES spectra were collected at room temperature in
the fluorescence mode with a solid state detector and processed by using the IFEFFIT
software with the Horae package (Athena and Artemis); the Fourierꢀtransformed curves were
fitted in real space with ꢁk = 3.0–11.0 Å−1, ꢁR = 1.2–3.2 Å for Pd (k2 weight).
Infrared (IR) spectra for nitrobenzene (NB) adsorption were measured on a BRUKER
TENSOR27 spectrometer with an MCT detector at a resolution of 2 cm−1 by accumulating
60 scans. Prior to the measurement, the sample was treated under highꢀvacuum (<1 Pa) to
remove any adsorbed water and other impurities.
2.3. Catalytic test
The hydrogenation of nitroarenes was conducted in a 30 mL stainless steel autoclave
equipped with a temperature and pressure control system. Typically, the autoclave was first
loaded with 5 mL methanol, 5 mg catalyst, and given amount of nitroarenes and kept at a
given temperature. 0.2 MPa of hydrogen was charged into the autoclave to start the
hydrogenation reaction and the reaction was then conducted at 0.2 MPa H2 under stirring
(300 rpm). After the reaction, the reactor was cooled in an iceꢀwater bath and the obtained
products were analyzed with a gas chromatograph (Shimadzu GCꢀ2014C with DBꢀ1
capillary column) with mꢀxylene as an internal standard. The turnover frequencies (TOF) are
calculated on the basis of total Pd atoms loaded on the catalyst by the equation
TOF = n0·x/(t·npd)
where n0 represents the initial molar amount of nitroarenes, x denotes the conversion of
nitroarenes after reaction for a certain time of t, and nPd is the molar amount of Pd present on
the catalyst charged in the autoclave reactor. Meanwhile, the turnover numbers (TON) are
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also evaluated by the equation
TON = n0·x/npd
To test the catalyst reusability, the spent catalyst sample was washed with acetone and
ethanol for several times to remove the organic substances retained on the catalyst surface
and then reꢀused in the next run. To consider the thermal stability and redox properties of
Pd/NSꢀCeO2, the catalytic performance of Pd/NSꢀCeO2ꢀ450 upon annealing in air at 300 °C
or 500 °C for 3 h (labeled as Pd/NSꢀCeO2ꢀ450 (300 °C) and Pd/NSꢀCeO2ꢀ450 (500 °C),
respectively) in the hydrogenation of nitrobenzene was also tested.
2.4. Computational method
Spinꢀpolarized DFT calculations were conducted by using the Vienna ab initio
simulation package (VASP) with projectorꢀaugmented wave (PAW) pseudoꢀpotentials.46–49
The
generalized
gradient
approximation
(GGA),
parameterized
by
Perdewt−Burke−Ernzerhof (PBE) functional was implemented to describe the electron
exchange and correlation energy.50 The planeꢀwave cutoff energy for the wave function was
set to 400 eV. All the structures were relaxed until the total energy was converged to 10−5 eV
and all the atomic forces were smaller than 0.05 eV Å−1. The DFT + U methodology was
used to describe the strongly localized Ce 4f electrons in the surface calculations;51,52 the
effective value of U was 4.5 eV.53
The CeO2 (111) surface was represented by a sixꢀlayer CeO2 slab, as reported by Zhang
and coꢀworkers.22 Large cells of 11.48×11.48×36.00 Å3 were used for CeO2, with periodic
boundary conditions. The vacuum layer between slabs was greater than 20 Å and the
MonkhorstꢀPack mesh kꢀpoint set was [3×3×1] for the surface calculations. The nitrobenzene
molecules and Pd clusters were simulated by using a cubic cell of 15×15×15 Å3, in which
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the Brillouin zone integration was treated for the Γ point only. The averaged adsorption
energies (Eads) were calculated by using the expression
Eads = Eadsorbate+surface – Eadsorbate – Esurface
where Eadsorbate+surface, Eadsorbate, and Esurface are the total energy of surface attached with
adsorbate, the energy of adsorbate, and the energy of clean surface, respectively. By this
definition, a more negative energy means that the surface is more easily covered by the
adsorbates.
3. Results and discussion
3.1. Synthesis of the 2D CeO2 nanosheets
Fig. 1A shows the SEM image of the asꢀsynthesized NSꢀCeO2ꢀ450 sample; it displays a
grapheneꢀlike morphology and the ceria sheets have a planar size of several micrometers.
The inset in Fig. 1A is a picture of NSꢀCeO2 suspended in water, showing the typical
Willis–Tyndall scattering effect; actually, the colloidal suspension is rather stable and can
stand for several days without forming any precipitates. The TEM image in Fig. 1B confirms
the layered 2D structure of NSꢀCeO2; the selectedꢀarea electron diffraction (SAED) pattern
shown in the inset displays several bright diffraction rings, suggesting that NSꢀCeO2 has a
polycrystalline structure. Highꢀresolution TEM image shown in Fig. 1C depicts that the 2D
NSꢀCeO2ꢀ450 sample is composed of a large number of small CeO2 slices attached to each
other, consistent with the result of SEAD. The atomic force microscopy (AFM) image and
the corresponding height profile shown in Fig. 1D illustrate that the thickness of NSꢀCeO2
sheets is about 3.7 nm; moreover, the fluctuation of the height profile reveals the porous
feature of 2D NSꢀCeO2. The physisorption results (Table S1, ESI†) illustrate that the pore
size of NSꢀCeO2 is in the range of 2–5 nm.
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{Fig. 1}
Above results indicate that 2D porous NSꢀCeO2ꢀ450 nanosheets can be prepared by first
depositing cerium precursors on the GO surface and then removing the GO template through
calcination in air. Further investigation reveals that the calcination temperature has a crucial
influence on the decomposition of cerium precursor, pyrolysis of the GOꢀCe complex,
ablation of GO, and construction of the 2D CeO2 sheets. As shown by the TGA curve in Fig.
S1, ESI,† the decomposition of the GOꢀCe complex starts at about 50 °C and ends at about
400 °C. The XRD patterns in Fig. S2A, ESI† indicate that the original GOꢀCe complex has a
lower crystallinity than GO. When the GOꢀCe complex is treated at 200 °C, it displays a
structure like rGO. When the GOꢀCe complex is calcined at 300 °C, a broadened CeO2
fluorite structure is observed, suggesting that the cerium nitrate precursor is decomposed to
CeO2 at this temperature, consistent with the TGA result. Moreover, the CeO2 diffraction
peaks in the XRD patterns become more intense with the increase of calcination temperature.
In comparison with bulk CeO2 (BꢀCeO2), NSꢀCeO2ꢀ450 (calcined at 450 °C) exhibits a slight
cell expansion; the cell parameter is extended from 5.410 Å for bulk CeO2 to 5.467 Å for
NSꢀCeO2ꢀ450, as illustrated in Fig. S2B and Table S1, ESI.† Meanwhile, as illustrated in Fig.
S3, ESI,† the NSꢀCeO2 samples calcined at 300–700 °C show the mesoporous structure with
the typical IV adsorptionꢀdesorption isotherms; the pore size increases slightly with the rise
of calcination temperature, at the expense of surface area and pore volume.
The SEM and TEM images shown in Fig. S4, ESI† confirm that all the NSꢀCeO2
samples have a porous and layered structure, but the pore size and layer thickness increase
with the rise of calcination temperature. Moreover, with the increase of calcination
temperature, the SEAD patterns are also transformed from several quite weak diffraction
rings to relatively ordered lattice arrangement, suggesting the structure transition of CeO2
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nanosheets from polycrystalline to single nanoꢀcrystals.
Besides using Ce(NO3)3, the 2D porous NSꢀCeO2 can also be obtained by using CeCl3
and Ce(NH4)2(NO3)6 as the precursors, as depicted in Fig. S5, ESI.† In addition, as
illustrated in Fig. S6, ESI,† a moderate mass ratio of GO to Ce(NO3)3, viz., 0.5−2, is
appropriate for the synthesis of 2D porous NSꢀCeO2 and the layer thickness of NSꢀCeO2
increases in general with the decrease of the GO/Ce(NO3)3 ratio. However, when the content
of GO is over low, for example, with a GO/Ce(NO3)3 mass ratio of 1/8, the layered
morphology is completely transformed into aggregated CeO2 nanoparticles.
Porous layered NSꢀCeO2 can also be prepared by calcining the GOꢀCe complex in O2, as
illustrated in Fig. S7, ESI;† however, if the GOꢀCe complex is calcined in Ar or H2, ultrafine
CeO2 nanoparticles with an average size of 1.7 nm decorated on the partially reduced
graphene oxide (rGO) are obtained.
3.2. Characterization of the 2D CeO2 nanosheets
The Ce 3d XPS spectra of the bulk CeO2 and NSꢀCeO2 samples shown in Fig. 2A
indicate that there exist two types of Ce species, viz., Ce3+ and Ce4+; however, various CeO2
samples are rather different in the relative refraction of two Ce species. In general, the Ce3+
species comes from oxygen vacancies and the content of surface Ce3+ species is directly
related to the concentration of oxygen vacancies in CeO2.54,55 As given in Table S2, ESI,†
the proportion of Ce3+ on the NSꢀCeO2ꢀ450 surface is 23.2%, much higher than the values of
15.0% in BꢀCeO2 and 15.7% in CeO2 nanorods reported by Zhang and coꢀworkers.22
Additionally, the proportion of Ce3+ on the NSꢀCeO2 surface deceases with the rise of the
calcination temperature.
{Fig. 2}
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In the Raman spectra (Fig. 2B), the peak at about 466 cm−1 is denoted as the active triply
degenerate mode (F2g), which is associated with the symmetrical stretching of Ce–O8
vibration unit related to Ce4+ species, whereas the weak shoulder (Ov) at about 600 cm−1 is
ascribed to the oxygen vacancies or the Ce3+ species.56,57 Obviously, the F2g mode peak of
NSꢀCeO2ꢀ450 shifts towards lower wave number (from 466.0 to 462.8 cm−1) and becomes
broader, in comparison with that of BꢀCeO2; such a shift is reported as mode softening due to
the presence of oxygen vacancies.56 As also given in Table S2, ESI,† the ratio of Ov/F2g for
NSꢀCeO2ꢀ450 is 0.261, much higher than the values of 0.149 for BꢀCeO2 and 0.146 for
NSꢀCeO2ꢀ850, meaning that NSꢀCeO2ꢀ450 is provided with a larger fraction of Ce3+ species,
viz., higher concentrated oxygen vacancies, than BꢀCeO2 and NSꢀCeO2 calcined at higher
temperatures.
In the O 1s XPS spectra (Fig. 2C), the asymmetric peaks of various ceria samples
illustrate that the surface oxygen species can be classified as Ce3+–O (533–531 eV) and
Ce4+–O (530–528 eV) bonded species.56 Obviously, NSꢀCeO2ꢀ450 exhibits much larger peak
area associated with Ce3+ and oxygen vacancies than BꢀCeO2 and NSꢀCeO2ꢀ850.
The H2ꢀTPR profiles of various ceria samples are displayed in Fig. 2D, to examine the
reducibility of various oxygen species. For NSꢀCeO2ꢀ450, the reduction of CeO2 starts at
335 °C and a sharp hydrogen consumption peak centered at 529 °C is observed, which is
attributed to the surface oxygen of CeO2. Meanwhile, a small and wide peak appears at
785 °C, especially for the NSꢀCeO2 samples calcined at high temperature, which is assigned
to the reduction of bulk oxygen in CeO2.58,59 For comparison, BꢀCeO2 shows a very small
peak at a slightly lower temperature (447 °C), but a broad peak centered at 820 °C. The
H2ꢀTPR results distinctly illustrate that the 2D layered NSꢀCeO2ꢀ450 sample affords much
more surfaceꢀreducible oxygen species than BꢀCeO2. Moreover, as also given in Table S3,
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ESI,† the content of surface oxygen species decreases markedly with the rise of calcination
temperature, consistent with the results of the XPS and Raman spectra.
The elemental analysis results of various NSꢀCeO2 samples are provided in Table S4,
ESI† and the residue of C and S after removing GO template is considered. Obviously, with
the increase of calcination temperature for removing the GO template, the surface O/Ce ratio
of NSꢀCeO2 is increased gradually, whereas the contents of residual C and S are reduced
considerably. For NSꢀCeO2ꢀ300 (calcined at 300 °C), the contents of C and S retained are
4.58 and 2.96 wt.%, respectively; however, when the NSꢀCeO2 samples was subjected to
calcination at 450 °C, the contents of residual C and S are decreased to about 0.4 wt.%, and
even less by calcining at higher temperatures.
3.3. Mechanism for the formation of 2D structure and universality of the EISA method
in the synthesis of various 2D metal oxides
Graphene is a hexagonal honeycomb and twoꢀdimensional sp2ꢀhybridized carbon
material.45 By using the modified evaporation induced selfꢀassembly (EISA) method, the
cerium precursors were first uniformly dispersed on the surface of graphene oxide (GO)
sheets, forming the amorphous GOꢀCe complex. As shown in Fig. S2A, ESI,† the GOꢀCe
complexes display very weak diffraction peaks, in comparison with GO and reduced GO
(rGO). Moreover, the characteristic peak of GO at 12.2°, which is typical for C (001)
corresponding to the interlayer space,60 is shifted to 14.6° in the GOꢀCe complex; it means
that the interlayer space is reduced from 0.704 nm for GO to 0.588 nm for the GOꢀCe
complex, as calculated by the Bragg’s formula. Furthermore, the BET surface area of the
GOꢀCe complex is declined to 10.8 m2 g−1 from 36.4 m2 g−1 of GO, as given in Table S1,
ESI.†
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The GOꢀCe complex presents as stacked sheets, due to the strong interaction between
GO and the cerium precursor, as shown in Figs. S4 and S8, ESI.† After calcining at 200 °C,
the GOꢀCeꢀ200 sample still shows the stacked layered structure, with the XRD pattern
similar to that of rGO, as displayed in Fig. S2A, ESI.† The characteristic peaks of CeO2
appear when the GOꢀCe complex is calcined at a temperature over 300 °C, accompanying
with the rapid fading of the characteristic peaks for the carbon layers. It can then be
concluded that at 200–300 °C, the GOꢀCe complex undergoes the decomposition of cerium
precursors and the pyrolysis and ablation of the carbon layer, in accordance with the TG and
XRD results illustrated in Figs. S1 and S2, ESI.† When the calcination temperature reaches
450 °C, the carbon layers is ablated completely and 2D NSꢀCeO2 nanosheets are formed
with relatively high crystallinity.
The formation of 2D CeO2 nanosheets can then be illustrated in Fig. 3. First, GO powder
was dispersed in the ethanol solvent under ultrasonic irradiation and the metal precursor was
then added, forming the stacked GOꢀCe complex. Next, the GOꢀCe complex was gradually
transferred to a large number of small CeO2 nanosheets during the calcination process,
accompanying with the decomposition of Ce precursor and the pyrolysis and ablation of the
carbon layers. Finally, the small CeO2 crumbs were assembled together, forming the
ultraꢀthin porous 2D CeO2 nanosheets.
{Fig. 3 & Fig. 4}
Furthermore, the modified EISA method used in this work has proved to be universal in
the synthesis of 2D nanosheets of various metal oxides. Besides NSꢀCeO2, a series of other
single and mixed metal oxides, including TiO2, ZrO2, CoOx, NiO, Al2O3, CuO, CuOꢀZrO2,
MnOxꢀAl2O3, FeOxꢀCoOx, Cr2O3ꢀZnO, CeO2ꢀZrO2, CeO2ꢀCuO, CeO2ꢀCoOx, CeO2ꢀLa2O3,
CeO2ꢀFeOx, CeO2ꢀMnOx, CeO2ꢀZnO, CeO2ꢀAl2O3, CeO2ꢀNiO, CeO2ꢀCr2O3, CeO2ꢀTiO2,
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MnOxꢀCeO2ꢀZrO2, and CuOꢀCeO2ꢀZrO2, were successfully prepared in nanosheets with
tunable pore structure and sheet thickness, through altering the metal precursors. The main
characteristic parameters of these 2D metal oxides, including estimated sheet thickness,
elemental composition, and residual content of C and S are provided in Table S5, ESI.†
As illustrated in Fig. 4 and Figs. S9–S17, ESI,† these metal oxides all exhibit the 2D
grapheneꢀlike morphology and the construction of 2D structure follows the same manner as
that for preparing NSꢀCeO2 in the EISA and later calcination processes. Huang and
coꢀworkers used the complexes of GO and metal hydroxides to prepare the 2D metal oxide
nanosheets and found that the transition of amorphous precursor to crystalline metal oxide
was critical to form the 2D structure;15 however, their method was less effective when
corresponding crystal hydroxide precipitates were formed at low temperature. In this work,
the complex of GO and metal precursors (GOꢀM) is directly converted to 2D metal oxides
without forming hydroxide precipitates in the modified EISA and subsequent calcination
processes, which could be a universal method to synthesize 2D grapheneꢀlike metal oxides.
3.4. Catalytic performance of Pd/NSꢀCeO2 in the hydrogenation of nitroarenes
Above results illustrate that the porous 2D CeO2 nanosheets (NSꢀCeO2) with a lateral
size of several micrometers and a thickness of 3.7 nm can be prepared by the modified EISA
method; moreover, the 2D CeO2 nanosheets have abundant surface Ce3+ species and oxygen
vacancies. To explore the potential of NSꢀCeO2 as a catalytic material, the catalytic
performance of Pd clusters supported on NSꢀCeO2ꢀ450 (Pd/NSꢀCeO2) in the hydrogenation
of nitroarenes was investigated. The Pd/NSꢀCeO2 catalyst is prepared through a liquid
deposition method; the TEM images shown in Fig. S18, ESI† illustrate that Pd is well
dispersed on the NSꢀCeO2 surface.
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As given in Table 1, the NSꢀCeO2ꢀ450 support alone is catalytically inactive in the
hydrogenation of nitrobenzene (NB); no products were detected even after reaction for 450
min (Entry 1). On the contrary, Pd/NSꢀCeO2ꢀ450 demonstrates excellent catalytic
performance (Entries 2 and 3); NB can be almost completely converted to aniline (AN) by
reaction at 25 °C for 1 h, with a turnover frequency (TOF) of 3010 h−1. A comparison of
current Pd/NSꢀCeO2 catalyst with some other catalysts previously reported in the literature in
terms of their performance is given in Table S6, ESI,† which indicates that Pd/NSꢀCeO2ꢀ450
is one of the most active catalysts known currently for the hydrogenation of NB at mild
conditions; at 40 °C, the TOF of Pd/NSꢀCeO2ꢀ450 (Table 1, Entry 4) even reaches 16873 h−1.
Moreover, NB can be completely converted to aniline even at freezing point (Entry 5) within
3 h, with a TOF of 357 h−1.
The evolution of the conversion of NB and selectivity to Nꢀphenylhydroxylamine (PHA)
and aniline (AN) with the reaction time for NB hydrogenation over Pd/NSꢀCeO2ꢀ450 is
illustrated in Fig. S19, ESI.† In the initial reaction stage, a small amount of PHA is formed
during the reaction. The content of PHA in the products reaches a maximum of about 3%
after reaction for 15 min and then decreases gradually with further prolonging the reaction
time; eventually, PHA disappear in the products after reaction for 45 min. Meanwhile, other
azo compounds such as azoxybenzene, azobenzene, and hydrazobenzene were not detected
during the reaction. These may suggest that the hydrogenation of NB over Pd/NSꢀCeO2
probably follows the direct reaction mechanism rather than the condensed one.61,62
{Table 1}
Besides, with NSꢀZrO2 and NSꢀNiO as the supports, the Pd/NSꢀZrO2 and Pd/NSꢀNiO
catalysts also display good activity in the hydrogenation of NB (Table 1, Entries 6 and 7),
but rather inferior to Pd/NSꢀCeO2. Meanwhile, the PdꢀIM/NSꢀCeO2 catalyst prepared by wet
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impregnation method (Entry 8) exhibits much lower activity than Pd/NSꢀCeO2 prepared by
liquid reduction, probably due to the poorer dispersion of Pd species in PdꢀIM/NSꢀCeO2; as
displayed in Fig. S20, ESI,† certain large Pd particles of 3.0−7.0 nm are present on the
PdꢀIM/NSꢀCeO2 surface. More catalytic tests illustrate that NSꢀCeO2ꢀ450 as a support is
rather superior to bulk CeO2 and other metal oxides for preparing the supported palladium
catalysts used in the hydrogenation of NB (Entries 9–12).
Further tests demonstrate that with hydrazine hydrate and isopropanol as the hydrogen
sources, Pd/NSꢀCeO2ꢀ450 is also very active for the hydrogenation of NB, as given in Table
S7, ESI;† NB can be completely converted to aniline under mild conditions by hydrazine
hydrate and isopropanol, though relatively higher temperature or longer reaction time may
be required than those for the hydrogenation of NB by hydrogen.
Meanwhile, the solvent used also has a significant influence on the hydrogenation of NB,
as given in Table S8, ESI.† The Pd/NSꢀCeO2ꢀ450 catalyst acts poorly for the hydrogenation
of NB in the aprotic solvents such as carbon dichloride and cyclohexane, but performs
excellently in the protic solvents like methanol, isopropanol and water. Especially, over the
Pd/NSꢀCeO2ꢀ450 catalyst, NB can be completely converted to aniline even in a solventꢀfree
environment, which could be an attractive green reaction process since the subsequent
separation and purification steps are greatly simplified.
Besides nitrobenzene, the catalytic performance of Pd/NSꢀCeO2ꢀ450 in the
hydrogenation of other nitroarenes with various substituted groups was also considered. As
given in Table 2, a complete conversion of pꢀmethylꢀnitrobenzene to pꢀmethylaniline is
achieved at 25 °C in 0.8 h (Entry 1), with a TOF of 3687 h−1, which is higher than that for
the hydrogenation of NB, probably due to the electronic donor effect of –CH3 group.
Meanwhile, the Pd/NSꢀCeO2ꢀ450 catalyst is also rather active for the hydrogenation of
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pꢀnitroaniline (Entry 2), pꢀnitrophenol (Entry 3), mꢀnitrobenzoic acid (Entry 4),
pꢀnitrobenzoic nitrile (Entry 5), and pꢀnitrobenzyl alcohol (Entry 6), where the conversion of
nitroarenes reaches over 99% in less than 2.5 h. Comparatively, Pd/NSꢀCeO2ꢀ450 is
relatively less active for the hydrogenation of pꢀnitrobenzaldehyde (Entry 7), oꢀnitroaniline
(Entry 8), and mꢀnitroaniline (Entry 9), where longer reaction time is required to achieve a
high conversion of nitroarenes. What is more exciting, a high selectivity to corresponding
anilines is generally observed, even for the hydrogenation of nitroarenes containing other
reducible substituent groups like −CHO, −COOH, and −CN.
{Table 2 & Fig. 5}
The reusability of the Pd/NSꢀCeO2ꢀ450 catalyst was then considered through a series of
consecutive tests; for each cycle, the spent catalyst sample was washed with acetone and
ethanol for several times to remove any organic substances retained on the catalyst surface
and then filtered and reꢀused in the next run. During the repetitive catalytic tests, as shown in
Fig. 5A, the conversion of NB and the selectivity to aniline both remain near 100% after 10
reaction cycles, suggesting that the Pd/NSꢀCeO2ꢀ450 catalyst owns also excellent stability
and reusability in the hydrogenation of NB to aniline, besides its high activity and selectivity.
3.5. Proposed factors contributing to the superior catalytic performance of Pd/NSꢀCeO2
To understand the catalytic behavior of Pd/NSꢀCeO2, the adsorption of NB on various
possible sites concerning both NSꢀCeO2 support and Pd cluster were evaluated by DFT
calculation. As demonstrated in Fig. 6A, for the bulk ceria with perfect CeO2 (111) surface,
NB is preferably adsorbed on the top Ce atoms via the oxygen atoms in nitro group, with the
adsorption energy of −0.21 eV, suggesting that NB molecules are weakly bonded on the
perfect CeO2 (111) surface. However, as illustrated above, NSꢀCeO2 surface is provided with
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abundant oxygen vacancies and Ce3+ species. On such a defective CeO2 (111) surface (Fig.
6B), NB can be strongly adsorbed on the NSꢀCeO2 surface through the interaction of the
oxygen atoms in nitro group and Ce atoms nearby an oxygen vacancy; the calculated
adsorption energy of NB on the defective CeO2 surface is −1.35 eV, far higher than that on
the perfect CeO2 surface (−0.21 eV). Meanwhile, the lengths of N–O bonds for NB adsorbed
on the defective CeO2 (111) surface are extended to 1.337 and 1.291 Å from the original
value of 1.242 Å in the NB molecule.
{Fig. 6}
Moreover, two models of Pd clusters (Pd6) supported on surfaceꢀdefect CeO2 (111) were
also considered for the adsorption of NB; that is, NB molecule can bind on Pd6 via two
oxygen atoms in nitro group or via the aromatic ring, as depicted in Figs. 6C and 6D. Two
adsorption modes have similar adsorption energy, viz., −0.75 eV vs. −0.77 eV, much lower
than the value of −1.35 eV for the adsorption of NB bound on the defective CeO2 (111)
surface. Meanwhile, the lengths of N–O bonds for NB adsorbed on the Pd6 cluster are only
lengthened to 1.282 and 1.279 Å, much shorter than those on the defective CeO2 (111)
surface.
The FTꢀIR spectrum of NB adsorbed on NSꢀCeO2 displays two intense bands near 1541
and 1421 cm−1, attributed to the framework vibration of benzene and asymmetrical stretch of
−NO2, respectively,22,63 as shown in Fig. S21, ESI,† also proving the strong interaction
between NB molecules and NSꢀCeO2 surface. As a result, NB molecules are more likely
adsorbed on the defective NSꢀCeO2 surface and the weakening of NꢀO bonds in NB
molecules adsorbed on NSꢀCeO2 is beneficial to the activation of NB reactant and the
subsequent hydrogenation reaction.
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The PdꢀK edge XANES and Pd 3d XPS spectra of Pd/NSꢀCeO2ꢀ450 (Figs. 5B, 5C and
5D and Tables S9 and S10, ESI†) further indicate that the Pd species loaded on NSꢀCeO2
surface with a low coordination number (5.6) is highly coordinativeꢀunsaturated and a large
fraction of Pd2+ species still exists in Pd/NSꢀCeO2ꢀ450 even after the reduction by NaBH4.
Similar result was also reported by Miller, Zou, and coꢀworker,64,65 which was ascribed to
the strong interaction between the Pd clusters and defective CeO2. The hydrogenation of NB
to aniline over the Pd/NSꢀCeO2ꢀ450 catalyst may take place between the NB molecule
preferentially adsorbed on the defective NSꢀCeO2 surface and the active hydrogen species
dissociated from the adjacent Pd clusters.
The effect of Pd loading on the conversion of NB in the initial stage of NB
hydrogenation over Pd/NSꢀCeO2ꢀ450 is illustrated in Fig. S22, ESI;† this series of tests were
carried out at the same molar ratio of NB substrate to Pd, through adjusting the weight of
catalysts loaded in the autoclave reactor according to their Pd loading. Obviously, the
catalytic efficiency of Pd species as the active component of Pd/NSꢀCeO2 decreases
considerably with the increase of Pd loading from 0.5 to 6wt.%. Such a phenomenon
supports the presumption that the hydrogenation of nitroarenes over Pd/NSꢀCeO2 occurs at
the interface between the Pd clusters and defective NSꢀCeO2 surface; a lower Pd loading is
generally able to give a higher Pd dispersion and to provide a larger fraction of active
interfacial sites between Pd and CeO2.
It was often reported that the activity of a supported metal catalyst in the hydrogenation
reactions declined substantially upon long term operation and repetitive utilization, or when
exposing to relatively high temperature.25,66 The Pd/NSꢀCeO2ꢀ450 catalyst in this work
exhibits excellent stability recyclability upon reusage; the conversion of NB and selectivity
to aniline both remain almost 100% even after 10 reaction cycles (Fig. 5A). To further assess
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its stability and tolerance to high temperature, the performance of Pd/NSꢀCeO2ꢀ450 catalyst
subjected to annealing at 300 or 500 °C for several times in the hydrogenation of NB was
also evaluated. As shown in Fig. 5A, the NB conversion over the Pd/NSꢀCeO2ꢀ450 (300 °C)
catalyst (upon annealing treatment at 300 °C) is reduced to 92.6%; luckily, it decreases
scarcely by further calcination treatments at the same temperature. However, the
Pd/NSꢀCeO2 catalyst is rather unstable upon calcination at higher temperature; the NB
conversion over the Pd/NSꢀCeO2ꢀ450 (500 °C) catalyst is gradually decreased to almost zero
upon calcination at 500 °C for 5 times (Fig. 5A).
The Pd 3d XPS (Fig. 5B) and PdꢀK edge XANES spectra (Figs. 5C and 5D) illustrate
that there exist both Pd2+ and Pd0 species in the active Pd/NSꢀCeO2ꢀ450 catalyst, whereas in
the Pd/NSꢀCeO2ꢀ450 (500 °C) catalyst, almost all the Pd species are transformed to Pd2+.
Such a variance of the Pd species in the valence state upon calcination treatment is also
proved by the H2ꢀTPR results, as illustrated in Fig. S23, ESI;† the low temperature reduction
peak of Pd species for the fresh Pd/NSꢀCeO2ꢀ450 catalyst is much weaker than those for the
Pd/NSꢀCeO2ꢀ450 (500 °C) and Pd/NSꢀCeO2ꢀ450 (300 °C) catalysts upon calcination
treatment. These results suggest that Pd0 is probably the catalytically active species and a
proper portion of Pd0 species in the Pd/NSꢀCeO2 catalyst is indispensable for the
hydrogenation of NB.22,67,68 In general, the deactivation of catalyst in the hydrogenation
reactions is caused by the sintering and poisoning of the active species as well as an
alteration of the valence state of the active component.69 In current work, the Pd clusters
with appropriate valence state are well stabilized in the defective 2D layered NSꢀCeO2
surface, which can endure repetitious reaction cycles of NB hydrogenation and treatment at
moderate temperature (< 300 °C).
On all accounts, the excellent catalytic performance of Pd/NSꢀCeO2ꢀ450 in the
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hydrogenation of NB should be ascribed to the synergy between the Pd species with
appropriate valence state and layered NSꢀCeO2 surface with abundant oxygen
vacancies;59,64,65 the layered defective ceria surface is able to stabilize the Pd clusters and
allow the Pd species having an appropriate valence state for the dehydrogenantion of NB.
The hydrogenation of NB to aniline over the Pd/NSꢀCeO2ꢀ450 catalyst may take place
between the NB molecule preferentially adsorbed on the defective NSꢀCeO2 surface and the
active hydrogen species dissociated from the adjacent Pd clusters. Moreover, the CeO2
nanosheets are enriched with surface defects and Ce3+ species, which can promote the
adsorption and activation of nitroarenes and then make the Pd/NSꢀCeO2ꢀ450 catalyst highly
active in the hydrogenation of nitroarenes.
4. Conclusions
A modified evaporation induced selfꢀassembly (EISA) method was developed to
synthesize the 2D grapheneꢀlike CeO2 nanosheets (NSꢀCeO2). The EISA method is proved to
be rather universal and a series of other single and mixed metal oxides, including TiO2, ZrO2,
CoOx, NiO, Al2O3, CuO, CeO2ꢀZrO2, CuOꢀZrO2, Cr2O3ꢀZnO, MnOxꢀAl2O3, FeOxꢀCoOx,
CeO2ꢀCuO, CeO2ꢀCoOx, CeO2ꢀLa2O3, CeO2ꢀFeOx, CeO2ꢀMnOx, CeO2ꢀZnO, CeO2ꢀAl2O3,
CeO2ꢀNiO, CeO2ꢀCr2O3, CeO2ꢀTiO2, MnOxꢀCeO2ꢀZrO2, and CuOꢀCeO2ꢀZrO2, are
successfully prepared in nanosheets with tunable pore structure and sheet thickness by using
this method.
The catalytic application of the grapheneꢀlike metal oxides was considered and the
results illustrate that the CeO2 nanosheets decorated with Pd clusters (Pd/NSꢀCeO2) exhibit
excellent catalytic performance in the hydrogenation of nitroarenes under mild conditions;
over it, nitrobenzene can be completely converted to aniline even at room temperature in a
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short time. Moreover, the Pd/NSꢀCeO2ꢀ450 catalyst owns excellent stability and reusability
in the hydrogenation of nitrobenzene to aniline, besides its high activity and selectivity; the
conversion of nitrobenzene and the selectivity to aniline both remain near 100% after 10
reaction cycles. What is more, a high selectivity to corresponding anilines is also observed
even for the hydrogenation of nitroarenes containing other reducible substituent groups like
−CHO, −COOH, and −CN.
A series of characterizations combined with DFT calculations demonstrate that the
NSꢀCeO2 nanosheets are enriched with surface defects and Ce3+ species, which promotes the
adsorption and activation of nitroarenes. The layered defective NSꢀCeO2 surface is also able
to stabilize the Pd clusters and allow the Pd species having an appropriate valence state for
the hydrogenation of nitroarenes. The synergy between the Pd species with appropriate
valence state and layered NSꢀCeO2 surface with abundant oxygen vacancies makes the
Pd/NSꢀCeO2ꢀ450 catalyst highly active in the hydrogenation of nitroarenes.
The present work brings forward a facile and effective approach to get grapheneꢀlike
metal oxides with superior catalytic performance; besides, the results obtained in this work
are also rather helpful to clarify the relationship between the 2D structure and performance
of the Pd/NSꢀCeO2 catalyst in the hydrogenation of nitroarenes.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgments
The authors thank Dr. J. Ma, Dr. S. Zhang and Prof. Y. Huang at Shanghai Synchrotron
Radiation Facility for the help with XAFS measurements as well as the financial support of
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National Natural Science Foundation of China (21403268, 21603254, 51602322 and
21703276), Youth Innovation Promotion Association of the Chinese Academy of Sciences
(No. 2016161), Strategic Program of Coalꢀbased Technology of Shanxi Province
(MQ2014ꢀ11, MQ2014ꢀ10), and the CAS/SAFEA International Partnership Program for
Creative Research Teams.
References
(1)
(2)
A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V.
Grigorieva, S. V. Dubonos and A. A. Firsov, Nature, 2005, 438, 197–200.
(3)
(4)
(5)
(6)
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V.
Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669.
S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A.
Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282–286.
W. Luo, Y. B. Wang, E. Hitz, Y. Lin, B. Yang and L. B. Hu, Adv. Funct. Mater., 2017,
27, 1701450–1701468.
S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta, H. R. Gutiérrez, T. F. Heinz, S.
S. Hong, J. Huang, A. F. Ismach, E. JohnstonꢀHalperin, M. Kuno, V. V. Plashnitsa, R.
D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones,
W. Windl and J. E. Goldberger, ACS Nano, 2013, 7, 2898–2926.
(7)
D. Deng, K. S. Novoselov, Q. Fu, N. Zheng, Z. Tian and X. Bao, Nat. Nanotechnol.,
26

Page 27 of 42
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C8TA02338G
2016, 11, 218–230.
(8)
(9)
A. J. Mannix, B. Kiraly, M. C. Hersam and N. P. Guisinger, Nat. Rev. Chem., 2017, 1,
0014–0027.
C. Tan, X. Cao, X. J. Wu, Q. He, J. Yang, X. Zhang, J. Chen, W. Zhao, S. Han, G. H.
Nam, M. Sindoro and H. Zhang, Chem. Rev., 2017, 117, 6225–6331.
(10) M. Xu, T. Liang, M. Shi and H. Chen, Chem. Rev., 2013, 113, 3766–3798.
(11) F. Ceballos and H. Zhao, Ultrafast Laser Spectroscopy of TwoꢀDimensional
Materials Beyond Graphene. Adv. Funct. Mater., 2017, 27, 1604509–1604522.
(12) S. P. Lau, L. J. Li and Y. Chai, Adv. Funct. Mater., 2017, 27, 1701403–1701404.
(13) J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A.
Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H.ꢀY. Kim, K. Lee,
G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C.
Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K.
Theuwissen, D. W. McComb, P. D. Nellist and V. Nicolosi, Science, 2011, 331,
568–571.
(14) S. Gao, X. Jiao, Z. Sun, W. Zhang, Y. Sun, C. Wang, Q. Hu, X. Zu, F. Yang, S. Yang,
L. Liang, J. Wu and Y. Xie, Angew. Chem., Int. Ed., 2016, 55, 698–702.
(15) Z. Huang, A. Zhou, J. Wu, Y. Chen, X. Lan, H. Bai and L. Li, Adv. Mater., 2016, 28,
1703–1708.
(16) L. Peng, P. Xiong, L. Ma, Y. Yuan, Y. Zhu, D. Chen, X. Luo, J. Lu, K. Amine and G.
Yu, Nat. Commun., 2017, 8, 15139–15148.
27

Journal of Materials Chemistry A
Page 28 of 42
View Article Online
DOI: 10.1039/C8TA02338G
(17) Y. Sun, Q. Liu, S. Gao, H. Cheng, F. Lei, Z. Sun, Y. Jiang, H. Su, S. Wei and Y. Xie,
Nat. Commun., 2013, 4, 2899–2906.
(18) Z. Sun, T. Liao, Y. Dou, S. M. Hwang, M. S. Park, L. Jiang, J. H. Kim and S. X. Dou,
Nat. Commun., 2014, 5, 3813–3821.
(19) S. Takenaka, S. Miyake, S. Uwai, H. Matsune and M. Kishida, J. Phys. Chem. C,
2015, 119, 12445–12454.
(20) J. F. Hartwig, Nature, 2008, 455, 314–322.
(21) T. E. Müller, K. C. Hultzsch, M. Yus, F. Foubelo and M. Tada, Chem. Rev., 2008, 108,
3795–3892.
(22) S. Zhang, C. R. Chang, Z. Q. Huang, J. Li, Z. Wu, Y. Ma, Z. Zhang, Y. Wang and Y.
Qu, J. Am. Chem. Soc., 2016, 138, 2629–2637.
(23) L. Chen, H. Chen, R. Luque and Y. Li, Chem. Sci., 2014, 5, 3708–3714.
(24) D. Guin, B. Baruwati and S. V. Manorama, Org. Lett., 2007, 9, 1419–1421.
(25) F. A. Harraz, S. E. ElꢀHout, H. M. Killa and I. A. Ibrahim, J. Catal., 2012, 286,
184–192.
(26) H. Wei, X. Liu, A. Wang, L. Zhang, B. Qiao, X. Yang, Y. Huang, S. Miao, J. Liu and
T. Zhang, Nat. commun., 2014, 5, 5634–5641.
(27) H. Yang, X. Cui, Y. Deng and F. Shi, ChemCatChem, 2013, 5, 1739–1743.
(28) G. C. Torres, E. L. Jablonski, G. T. Baronetti, A. A. Castro, S. R. de Miguel, O. A.
Scelza, M. D. Blanco, M. A. Pena Jiménez and J. L. G. Fierro, Appl. Catal., A, 1997,
28

Page 29 of 42
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C8TA02338G
161, 213–226.
(29) S. R Miguel, J. I. Vilella, E. L. Jablonski, O. A. Scelza, C. S. Lecea and A.
LinaresꢀSolano, Appl. Catal., A, 2002, 232, 237–246.
(30) K. Min, J. S. Choi, Y. M. Chung, W. S. Ahn, R. Ryoo and P. K. Lim, Appl. Catal., A,
2008, 337, 97–104.
(31) M. Oubenali, G. Vanucci, B. Machado, M. Kacimi, M. Ziyad, J. Faria, A.
RaspolliꢀGaletti and P. Serp, ChemSusChem, 2011, 4, 950–956.
(32) C. H. Li, Z. X. Yu, K. F. Yao, S. f. Ji and J. Liang, J. Mol. Catal. A: Chem., 2005, 226,
101–105.
(33) Z. Sun, Y. Zhao, Y. Xie, R. Tao, H. Zhang, C. Huang and Z. Liu, Green Chem., 2010,
12, 1007–1011.
(34) P. Chen, F. Yang, A. Kostka and Wei Xia, ACS Catal., 2014, 4, 1478–1486.
(35) R. Nie, J. Wang, L. Wang, Y. Qin, P. Chen and Z. Hou, Carbon, 2012, 50, 586–596.
(36) J. Sun, Y. Fu, G. He, X. Sun and X. Wang, Catal. Sci. Technol., 2014, 4, 1742–1748.
(37) S. Basu, M. Mapa, C. S. Gopinath, M. Doble, S. Bhaduri and G. K. Lahiri, J. Catal.,
2006, 239, 154–161.
(38) Y. M. Yamada, Y. Yuyama, T. Sato, S. Fujikawa and Y. Uozumi, Angew. Chem., Int.
Ed., 2014, 53, 127–131.
(39) F. CárdenasꢀLizana, S. GómezꢀQuero, A. Hugon, L. Delannoy, C. Louis and M. A.
Keane, J. Catal., 2009, 262, 235–243.
29