Highly dispersed Pd nanoparticles supported on nitrogen-doped graphene with enhanced hydrogenation activity

Article abstract of DOI:10.1039/c5ra12243k

Pd nanoparticles supported on nitrogen-doped graphene (NG) were prepared as hydrogenation catalysts. Different nitrogen sources (ethylenediamine, ammonia, and urea) were employed to synthesize NG using hydrothermal treatment under mild conditions. The as-made samples were characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, elemental analysis, nitrogen adsorption-desorption, X-ray diffraction, and X-ray photoelectron spectroscopy. Remarkably improved dispersion of Pd nanoparticles was observed when nitrogen was introduced into the graphene structure. These NG-supported Pd catalysts showed enhanced catalytic hydrogenation activities owing to the superior dispersion of Pd. In the hydrogenation of different olefins, perfect turnover frequencies were obtained over the NG-supported Pd catalyst with urea as the nitrogen source.

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Highly dispersed Pd nanoparticles supported on nitrogen-doped
graphene with enhanced hydrogenation activity
Ping Liu,*a Gen Li,a Wan-Ting Chang,a Meng-Yao Wu,a Yong-Xin Li*a and Jun Wangb
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Pd nanoparticles supported on nitrogenꢀdoped graphene (NG) were prepared as hydrogenation catalysts.
Different nitrogen sources (ethylenediamine, ammonia, and urea) were employed to synthesise NG by
hydrothermal treatment at a mild condition. The asꢀmade samples were characterized by transmission
electron microscopy, scanning electron microscopy, Raman spectroscopy, elemental analysis, nitrogen
10 adsorptionꢀdesorption, Xꢀray diffraction, and Xꢀray photoelectron spectroscopy. Remarkably improved
dispersion of Pd nanoparticles were observed when nitrogen was introduced into the graphene structure.
These NGꢀsupported Pd catalysts showed enhanced catalytic hydrogenation activities owing to the
superior dispersion of Pd. In the hydrogenation of different olefins, perfect turnover frequencys were
obtained over NGꢀsupported Pd catalyst with urea as the nitrogen source.
45 carbon materials is more effective to anchor metal nanoparticles
depending on the interaction between N species and metal
nanoparticles.17ꢀ20 Significantly, nitrogen doping also introduces
structural defects and alters the electronic structure of graphene,
which may promote the catalytic process.21
Some methods have been reported to synthesize nitrogenꢀ
doped graphene (NG), such as chemical vapour deposition,22
thermal annealing graphene oxide with NH3,23 arc discharge,24
and hydrothermal method.21 The first three methods all require
rigorous conditions or special instruments. Hydrothermal
55 treatment is deemed to be a facile and lowꢀcost process to prepare
NG. Recently, Yu et al.25 developed a hydrothermal reaction to
synthesize NG with graphene oxide and organic amine as
precursors at a much low temperature. The obtained NG showed
a remarkably enhanced performance for ultrafast supercapacitor.
60 Nevertheless, it has not been studied about whether the NG
synthesized by the facile hydrothermal method can be used as an
effective support to highly disperse the metal nanoparticles and
further improve the catalytic activity.
To circumvent this problem, in this work, we synthesize NG
65 with different nitrogen sources through hydrothermal process,
and use the NG as support to load Pd nanoparticles. Influences of
the properties of NG on the state of Pd particles and the catalytic
hydrogenation performances are in detail investigated.
15 1. Introduction
Metals, especially noble metals Pt, Pd, Rh, and Au, loaded on the
support are widely used as catalysts in catalytic hydrogenation
reactions because of their high activity, superior stability and
reusability.1 As for the supported catalysts, the properties of
20 supports would greatly influence the catalytic performances from
different perspectives such as the state of metal nanoparticles, the
adsorption and/or diffusion of reactant and products.2,3 Generally,
an ideal support should promote the dispersion of metals and
accelerate the diffusion of reactant and products. Although many
25 supports (e.g. Al2O3,4 SiO2,5 activated carbon,6 zeolite,7 SBAꢀ158
and carbon nanotube (CNTs)9) have been studied during the past
few decades years, the researchers never stop seeking the new
materials to further improve the activity of loaded metal catalysts.
As an emerging member of carbon materials family, graphene
30 with a unique twoꢀdimensional (2D) honeycomb lattice structure
shows unusual properties, including excellent electrical and
thermal conductivity, extraordinary thermal and mechanical
stability, and extremely high theoretical specific surface area
(theoretical 2620 m2·g−1).10,11 These properties make graphene to
35 be a potential ideal support for loading metal nanoparticles.12,13
However, in the process of loading metal catalyst on the
graphene, some issues are exposed such as the uneven dispersion
of metal active sites, and loss and/or aggregation of metal
nanoparticles due to the relatively weak interaction between
40 graphene and metal.14 In order to solve these problems, some
surfactants,15 ligands,14 and polymers16 have been applied to
stabilize the nanoparticles. Unfortunately, these additives are
usually expensive and may severely restrict the catalytic
activities. In comparison, nitrogen doping into the framework of
50
2. Experimental
70 2.1 Preparation of catalysts
2.1.1 Synthesis of NG
Graphene oxide (GO) was prepared via ultrasonic exfoliating the
graphite oxide obtained by the modified Hummer’s method.26 NG
was synthesized through a facile hydrothermal method using GO
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3.1.1 SEM
as the precursor. In
a typical process, nitrogen source
As shown in Fig. 1, GO gave large sheets with numerous
wrinkles. When GO was hydrothermally treated in different
aqueous solutions of above nitrogen sources, threeꢀdimensional
60 porous network with dozens of nanometers pores were obtained.
The structures were relatively uniform for various Nꢀdoped
graphene samples with 3.7 mmol of Nꢀinputting amount. It can be
speculated that the threeꢀdimensional microstructures can avoid
the stacking of graphene sheets which is a difficult problem for
65 the application of graphene in heterogeneous catalysis. Pd/NGꢀ
AWꢀ2.2 gave the similar topological structure to that of Pd/NGꢀ
AWꢀ3.7, while different result was obtained on Pd/NGꢀUreaꢀ2.2.
For the SEM image of Pd/NGꢀUreaꢀ2.2, part of graphene sheets
had not been reconstructed, and still maintained the original
(ethylenediamine, ammonia, or urea) was gradually added into 40
mL of GO suspension (2 mg·mL−1) in deionized water. The
mixture was stirred at room temperature for 10 min and
subsequently moved to a 50ꢀmL Teflonꢀlined autoclave. After
hydrothermal treatment at 180 °C for 12 h, the mixture was
naturally cooled to room temperature, and the product was
obtained by centrifugation, followed by washing with deionized
water and drying in vacuum. The prepared NG was denoted as
5
10 NGꢀxꢀy, x indicates the nitrogen source (EDA: ethylenediamine,
AW: ammonia, Urea: urea) and y indicates the micromolar
quantity of the input of nitrogen element.
2.1.2 Synthesis of Pd/NG
40 mg NG, 0.2 mL PdCl2 aqueous solution (0.02 mol·L−1), and
15 40 mL deionized water were added into a roundꢀbottom flask. 70 stacking state. The possible explaination is that the much low
After mixing uniformly, 1 mL hydrazine hydrate was then
dropwise added with magnetically stirring for 30 min.
Subsequently, the mixture was centrifuged, and the obtained solid
catalyst was fully washed with deionized water and dried in
20 vacuum. For comparison, Pd supported on graphene without
nitrogen doping (denoted by Pd/rGO) was prepared by the same
process with GO as the support precursor. The specifications of
supported Pd catalysts including Pd loadings are summarized in
Supporting Information, Table S1.
concentration of urea (2.8×10ꢀ2 mol·L−1, Table S1 in Supporting
Information) for the preparation of Pd/NGꢀUreaꢀ2.2 restricts the
speed or degree of Nꢀdoping.
25 2.2 Characterization
Scanning electron microscopy (SEM) measurements were carried
out on a field emission scanning electron microanalyzer (JEOLꢀ
6700F). Nitrogen adsorptionꢀdesorption were recorded at −196
°C on a Micromeritics ASAP 2020 analyzer with pretreatment of
30 samples at 150 °C for 4 h. Raman spectra were recorded on a
Raman spectrometer (LabRAM HR EvoLution) with the green
line of an Arꢀion laser of 532 nm. Xꢀray photoelectron
spectroscopy (XPS) were recorded on a RBD upgraded PHIꢀ
5000C ESCA system (Perkin Elmer) with Mg or Al Kα radiation.
35 Elemental analysis was measured on a PerkinElmer EA2400 II
instrument to determine the C, N, and H content of the samples.
Transmission electron microscopy (TEM) and energy dispersive
Xꢀray spectroscopy (EDS) measurements were performed using a
JEOL JEMꢀ2100 high resolution transmission electron
40 microscope. Xꢀray diffraction (XRD) patterns were measured on
a D/max 2500 PC Xꢀray diffractometer using Cu Kα radiation
with a scanning range of 5ꢀ60 °. Pd dispersion measurements
were conducted using a JAPAN BELCAT Analyzer using a
pulsed technique of hydrogen chemisorptions at 50 °C.
75 Fig. 1 SEM images of GO and various nitrogenꢀdoped graphene samples.
3.1.2 N2 adsorption-desorption
In the results of nitrogen adsorptionꢀdesorption measurement
(Table 1), GO exhibited a low BET surface area of 87 m2·g−1
with scarcely any pore volume. It is possible that GO sheets got
80 stacking when the water was removed during the pretreatment
under vacuum and 150 °C. Both high BET surface areas and pore
volumes for Nꢀdoped graphene samples were received, owing to
the threeꢀdimensional porous network. It further illustrates that
the threeꢀdimensional microstructures of Nꢀdoped graphenes are
85 enough stability under the conditions of pretreatment. Comparing
different Nꢀdoped samples, NGꢀUreaꢀ3.7 showed the highest
BET surface area (608 m2·g−1) and pore volume (402 mm3·g−1).
45 2.3 Catalytic test
The catalytic performances were tested in the hydrogenation of
cyclohexene. Typically, 0.01 g catalyst, 0.5–2 mL cyclohexene,
and 10 mL ethanol were added into a stainless steel autoclave.
After replacing the air in the autoclave with argon, 1 MPa H2 was
50 injected, and the catalytic tests were then performed at 30 °C for
1 h. The products were analyzed by GC (SPꢀ6890) with a OVꢀ
101 capillary column. The recovered catalyst was washed with
ethanol and employed in the next run.
Table 1 BET surface areas and pore volumes for various samples.
Samples
GO
BET surface area/(m2·g−1) Pore volume /(mm3·g−1)
87
―
NGꢀEDAꢀ3.7
NGꢀAWꢀ3.7
NGꢀUreaꢀ3.7
414
429
608
268
308
402
3. Results and discussion
55 3.1 Characterization of various catalysts
2
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3.1.3 XPS and elemental analysis
25
The chemical composition (contents of C, O, and N) of GO
and various Nꢀdoped graphene samples with detailed amounts of
different nitrogen species obtained by XPS analysis were
displayed in Table 2. Compared to the contents of C, O and N
obtained by the elemental analysis (Supporting Information,
As shown in Fig. 2, the spectra reveal that the main chemical
components were C 1s, N 1s, O 1s in NGꢀUreaꢀ3.7, with a clear
N 1s peaks around a binding energy of 400 eV. The amount of
nitrogen incorporated in NGꢀUreaꢀ3.7 was found to be 4.6 at%,
indicating successful Nꢀdoping. The C 1s spectra of GO and NGꢀ
Ureaꢀ3.7 were separated into four peaks (Fig. 2), at around 284.4,
286.3, 287.3 and 288.7eV, corresponding to CꢀC, CꢀOH, CꢀOꢀC
and O=CꢀO groups, respectively (Scheme 1).27 For the result of
10 XPS, GO contained abundant oxygenꢀcontaining groups, while
much few oxide functionalities were observed on NGꢀUreaꢀ3.7.
Moreover, the N 1s spectrum of NGꢀUreaꢀ3.7 was also separated
into four peaks, at 398.6, 399.5, 400.2, and 401.4 eV. These
peaks were successively ascribed to pyridinic N, amine moieties
15 (e.g., –NH– and –NH2), pyrrolic N, and graphitic N species
(Scheme 1).25 Other NG samples (NGꢀEDAꢀ3.7, NGꢀAWꢀ2.2 and
NGꢀAWꢀ3.7) showed similar XPS spectra to that of NGꢀUreaꢀ3.7
(Supporting Information, Fig. S1). It implied that drastic
reduction of GO occurred during the hydrothermal treatment,
20 whilst four kinds of nitrogen were doped into the graphene
structure.
5
30 Table S2), O and N amounts decreased, while C amount
increased for the results of XPS. It is possible that some C
pollution on the surface of samples influence the XPS analysis.
Nevertheless, the general tendencies of Nꢀdoping situation were
consistent for the two results of XPS and elemental analysis.
35 Through hydrothermal treatment in the aquous solution of
ethylenediamine, ammonia, or urea, different nitrogen species
and contents were doped into the framework of graphene. Among
these Nꢀdoped graphenes, NGꢀEDAꢀ3.7 had the most amounts of
nitrogen (6.8 at%); NGꢀUreaꢀ3.7 and NGꢀAWꢀ2.2 had similar
40 contents of nitrogen (4.6 and 4.7 at%, respectively) which is
slightly lower than that in NGꢀAWꢀ3.7 (4.9 at%). For the
different nitrogen species, relatively more amine moieties (2.1
at%) were observed in NGꢀEDAꢀ3.7, while less amine moieties
(0.9 at%) were included in NGꢀAWꢀ2.2. NGꢀUreaꢀ3.7 gave a
45 much less amounts of Graphitic N (0.6 at%). As we all known,
different nitrogen species show different behaviors. For the four
kinds of nitrogen, pyridinic N and graphitic N are considered to
be responsible for stabilizing the metal nanoparticles.28,29 In this
work, NGꢀEDAꢀ3.7 possessed the most total amount of pyridinic
50 N and graphitic N, while NGꢀUreaꢀ3.7 had the least amount. In
addition, compared to the oxygen amount of 42.6 at% for GO,
less than 30 at% of oxygen were obtained on all Nꢀdoped
graphenes. It is because that the parts of oxygenꢀcontaining
functional groups are removed during the Nꢀdoping process (Fig.
55 2). In comparison, NGꢀUreaꢀ3.7 exhibited the most content of
oxygen (23.7 at%), while NGꢀEDAꢀ3.7 gave the least amount of
oxygen (17.3 at%).
C 1s
N 1s
CO2H
OH
O
O
H2N
HN
286.3 eV
399.5 eV
401.4 eV
N
287.3 eV
284.4 eV
N
O
OH
400.2 eV
398.6 eV
N
N
CO2H
288.7 eV
CO2H
Scheme 1 A possible structure of NG. The data correspond to the binding
energies as obtained from the C and N 1s spectra.
C 1s
N 1s
O 1s
GO
C 1s
NG-Urea-3.7
C 1s
GO
C 1s
NG-Urea-3.7
O 1s
N 1s
290 288 286 284 282
Binding energy (eV)
294 292 290 288 286 284 282 406 404 402 400 398 396 394
Binding energy (eV)
600
500
400
300
200
Binding energy (eV)
60
Fig. 2 XPS survey scan and XPS C and N 1s spectra for GO and NGꢀUreaꢀ3.7.
According to the reported Nꢀdoping mechanism in the
literatures,25,30 the possible Nꢀdoping reaction pathway is
discussed as follows. When the gaphene oxide (GO) is treated
acid species on GO to form the amideꢀlike structure. The amideꢀ
like structure will convert to the stable aromatic structure or
imine by decarbonylation. During this process, GO is reduced
simultaneously due to the reaction between nitrogen source and
under
hydrothermal
condition,
the
nitrogen
source
65 (ethylenediamine, ammonia, and urea) will react with carboxylic 70 other oxygenꢀcontaining groups. Then, the reduced graphene
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oxide sheets will assemble into threeꢀdimensional porous network
(SEM results) with the πꢀπ interaction.31 Moreover, the amine
moieties on the graphene sheets will hinder their stacking by the
forming of hydrogen bond.
5
Table 2 Chemical composition of GO and various Nꢀdoped graphenes.
Sample
C/O/N (at%)
Pyridinic N (at%)
Amine moieties (at%)
Pyrrolic N (at%)
Graphitic N (at%)
GO
56.4/42.6/—
74.9/17.3/6.8
73.6/20.2/4.7
73.4/19.7/4.9
69.5/23.7/4.6
—
1.7
1.5
1.2
1.4
—
2.1
0.9
1.3
1.1
—
1.7
1.2
1.4
1.5
—
1.3
1.1
1.0
0.6
NGꢀEDAꢀ3.7
NGꢀAWꢀ2.2
NGꢀAWꢀ3.7
NGꢀUreaꢀ3.7
Contents of C, O and N and contents of different N kinds were obtained by the XPS analysis.
smaller Pd particles were obtained on various Nꢀdoped graphenes
(Fig. 4). The results were consistent with the Pd dispersions
shown in Table S3 (Supporting Information). Pd/rGO showed a
35 much low Pd dispersion of 12.1% due to the serious aggregation.
Pd supported NG catalysts all gave much higher Pd dispersions
beyond 60%. Obviously, nitrogen introducing greatly promotes
the dispersion of Pd depending on the increased surface area
(Table 1) and nitrogen species (Table 1). It is considered to be
40 that some electron transfer and coordination between Pd and Nꢀ
doped graphene can prevent the metal from migrating and
aggregating.33,34 During the Pd/NG catalysts treated with different
nitrogen sources, ureaꢀtreating samples including Pd/NGꢀUreaꢀ
2.2 and Pd/NGꢀUreaꢀ3.7 both showed uniform and wellꢀdispersed
45 Pd nanoparticles (NPs). Uneven Pd NPs distributions with some
aggregations were exhibited on the NG supports treated with
ethylenediamine and ammonia. In comparison, Pd/NGꢀUreaꢀ3.7
showed the ultrasmall Pd particles with about 2 nm (Fig. 4 F),
while Pd/NGꢀEDAꢀ3.7 gave the relatively large size of Pd
3.1.4 Raman
10 As shown in Fig. 3, two main peaks were obtained for all Raman
spectra of GO and Nꢀdoped graphenes, corresponding to a
disorderꢀinduced D band and a sp2 hybridized G band at about
1350 and 1585 cm−1, respectively.32 When nitrogen elements
were introduced into the framework of graphene, ID/IG had more
15 or less increase. It indicates that more defect sites form on the
graphene sheets or the size of the sp2 hybridized carbon atom
clusters reduces during the process. Significantly, the ID/IG
decreased gradually with the increase of Nꢀdoping contents
(Table 1). NGꢀEDAꢀ3.7 with the most nitrogen amounts showed
20 the lowest ID/IG ratio of 1.03, while NGꢀAWꢀ2.2 gave the highest
ID/IG ratio of 1.25. It implies that, with increasing the Nꢀ
introducing contents, more sp2 hybridized carbon atoms are
recovered along with the chemical reduction of GO during the
hydrothermal treatment.
D G
50 particles with 10–12 nm (Fig. 4 B). Combining with the results of
elemental analysis and Raman, it can be speculated that the
oxygen functional groups and defect sites on the graphene also
play important roles in the dispersion of Pd NPs. Compared to
Pd/NGꢀEDAꢀ3.7, despite with less amounts of pyridinic N and
55 graphitic N, Pd/NGꢀUreaꢀ3.7 gave a higher dispersion of Pd NPs
owing to more oxygen groups (Table 2) and defect sites (Fig. 3).
I /I =1.02
D G
A
B
I /I =1.03
D G
I /I =1.25
D G
C
3.1.6 XRD
As shown in Fig.5, a distinct diffraction peak at 11.4 º was
60 observed on GO assigned to the (001) plane of graphene oxide,
indicating the successful oxidation of graphite.35 After
I /I =1.08
D G
D
E
F
I /I =1.22
D G
hydrothermal treatment with urea at 180 °C, the characteristic
peak at 11.4 º disappeared and new peaks at 24.8 º (assigned to
the (002) plane of graphite) appeared for NGꢀUreaꢀ3.7 and
65 Pd/NGꢀUreaꢀ3.7 (JCPDS card no. 41ꢀ1487). It suggests that GO
was reduced during the Nꢀdoping process. Moreover, Pd/NGꢀ
Ureaꢀ3.7 showed a low and broad diffraction peak at 39.7 º,
assigning to the (111) plane of Pd (JCPDS card no. 65ꢀ2867).
For the EDS results of NGꢀUreaꢀ3.7 and Pd/NGꢀUreaꢀ3.7
70 (Supporting Infromation, Fig. S2), C and O elements existed in
NGꢀUreaꢀ3.7, and additional Pd element existed in Pd/NGꢀUreaꢀ
3.7. The peak of N element is overlapped by C peak because of
much nearing between the two elements peaks and the strong
peak of C element.
I /I =1.11
D G
0
500 1000 1500 2000 2500 3000 3500
-1
Raman shift (cm )
25
Fig. 3 Raman spectra of GO (A) and nitrogenꢀdoped graphene samples
(BꢀF). (B) NGꢀEDAꢀ3.7; (C) NGꢀAWꢀ2.2; (D) NGꢀAWꢀ3.7; (E) NGꢀ
Ureaꢀ3.7; (F) NGꢀUreaꢀ5.2. ID/IG: the intensity ratio of D and G band.
3.1.5 TEM
30 Under the same preparation conditions, large Pd particles with
dozens of nanometers were observed for Pd/rGO, while much
4
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Fig. 4 TEM images and Pd particle size distributions of various Pdꢀloaded catalysts. (A) Pd/rGO; (B) Pd/NGꢀEDAꢀ3.7; (C) Pd/NGꢀAWꢀ2.2; (D) Pd/NGꢀ
AWꢀ3.7; (E) Pd/NGꢀUreaꢀ2.2; (F) Pd/NGꢀUreaꢀ3.7.
and 1 h. Much low activities were obtained over various NG
GO(001)
samples including NGꢀEDAꢀ3.7, NGꢀAWꢀ3.7, and NGꢀUreaꢀ3.7.
It suggests that Nꢀdoped graphene cannot act as the effective
active center for the catalytic hydrogenation of cyclohexene
20 under these reaction conditions. Nevertheless, the activity was
significantly improved when Nꢀdoped graphenes were employed
a
G(002)
to support Pd catalysts. Among these Pd/NG catalysts with the
G(100)
same nitrogen inputting amount, Pd/NGꢀEDAꢀ3.7 exhibited a
b
relatively low activity of 58.2% at 50
25 and Pd/NGꢀUreaꢀ3.7 gave complete transformation of
cyclohexene. When the reaction temperature was reduced to 30
C, 61.0% of conversion with 6530 mol·(molPd·h)−1 of TOF was
°C, while Pd/NGꢀAWꢀ3.7
Pd(111)
a
c
°
0
10
20
30
40
50
60
70
obtained over Pd/NGꢀAWꢀ3.7. For Pd/NGꢀUreaꢀ3.7, four times
of the substrate (2 mL cyclohexene) was employed to make the
30 reaction at the kinetically controlled step. At this case, a 58.9% of
conversion with an extremely high TOF of 27463
2θ
/
degree
5
Fig. 5 XRD patterns of GO (a), NGꢀUreaꢀ3.7 (b), and Pd/NGꢀUreaꢀ3.7 (c).
mol·(molPd·h)−1 was achieved at 30
°C. Catalytic hydrogenation
3.2 Catalytic studies
activities of supported Pd catalysts greatly depend on the
dispersion state of Pd NPs. Actually, the dispersion state of Pd
35 determines the number of exposed activity centers. From the
results of TEM and Pd dispersion measurement, it can be
confirmed that there are much less activity centers on Pd/rGO and
more activity centers on Pd/NG catalysts. Therefore, the low TOF
of Pd/rGO is because of the agglomeration of Pd particles, and
40 the enhanced activities of various Pd/NG catalysts are precisely
attributed to the promoted dispersion of Pd NPs after Nꢀdoping
3.2.1 Catalytic performances
The hydrogenation of cyclohexene was chosen as the probe
10 reaction to test the catalytic performances of various catalysts
because of its simple product. As listed in Table 3, 100% of
selectivity to cyclohexane was obtained under the mild reaction
conditions over all catalysts without ringꢀopening, isomerization,
or other products. Pd/rGO showed a low conversion of 19.0%
15 with only 1718 mol·(molPd·h)−1 of TOF under 1.5 MPa, 50
°C
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(Fig. 4). During these Nꢀdoped catalysts, the superior Pd
dispersion of 90.1% with much high density of active sites for
Pd/NGꢀUreaꢀ3.7 leads to the excellent catalytic activity.
Table 3 Comparison of catalytic activities of Pd/NG catalysts in the hydrogenation of cyclohexene.
TOFa
(mol·(molPd·h)−1)
1718
Catalyst
H2 pressure (MPa)
Conversion (%)
Selectivity (%)
Reaction temperature (°C)
Pd/rGO
NGꢀEDAꢀ3.7
NGꢀAWꢀ3.7
NGꢀUreaꢀ3.7
Pd/NGꢀEDAꢀ3.7
Pd/NGꢀAWꢀ3.7
Pd/NGꢀAWꢀ3.7
Pd/NGꢀUreaꢀ3.7
Pd/NGꢀUreaꢀ3.7 b
1.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
50
50
50
50
50
50
30
50
30
19.0
3.1
5.3
100
100
100
100
100
100
100
100
100
—
—
—
5871
>10705
6530
>11657
27463
4.8
58.2
100
61.0
100
58.9
5
Reaction conditions: 0.01 g catalyst, 0.5 mL cyclohexene, 10 mL ethanol, 1 h.
a Moles of cyclohexene converted per mole of Pd per hour; b 2 mL cyclohexene.
3.2.2 Effect of N-inputting amount
The structural morphology, elemental content of graphene, and
50 of Pdꢀsupported on Nꢀdoped graphene in heterogeneous
hydrogenation system.
10 the Pd dispersion are all changed during Nꢀdoping process
(Section 3.1). The above aspects of a catalyst may greatly
influence its catalytic performance. Therefore, in order to get a
high activity, an appropriate Nꢀdoping amount is required. As
shown in Fig. 6, 2.2 and 3.7 mmol are the optimum Nꢀinputting
15 amounts for Pd/NGꢀAW and Pd/NGꢀUrea, respectively. About
100% conversion of cyclohexene over the two corresponding
100
90
80
70
60
50
catalysts were obtained at 30 °C. Comparing the actual contents
of N element for NG with different Nꢀdoping conditions in
Tables 2 and S4 (Supporting Information), with the increase of
20 Nꢀinputting amount, the actual Nꢀdoping content increased and
oxygenꢀcontaining groups reduced gradually. For different N
species, there were unified change rule with the increase of Nꢀ
inputting amount. During the NGꢀAW and NGꢀUrea samples,
NGꢀAWꢀ2.2 and NGꢀUreaꢀ5.2 had the most amounts of Pyridinic
25 N and Graphitic N, respectively. The catalytic activity is closely
related with the dispersion state of Pd which depends on several
factors, such as N species, oxygenꢀcontaining groups, and defect
sites on graphene sheets. Considering all the factors, in general,
too little or too much Nꢀinputting amount all result in low
30 catalytic activities. Too little nitrogen amount would be not
enough to stable the Pd NPs (Tables 2 and S4), while too much
nitrogen amount would lead to less defect sites (Fig. 3) and
oxygenꢀcontaining groups (Tables 2 and S4) which have some
contributions to the dispersion of metal (Fig. 4).36 In this work,
35 more amounts of Pyridinic N and Graphitic N for NGꢀAWꢀ2.2
and more amounts of oxygenꢀcontaining groups and defect sites
for NGꢀUreaꢀ3.7 maybe the main reasons for the higher Pd
dispersions and superior catalytic activities over their
corresponding catalysts. In order to further filtrate the highly
40 efficient catalyst between Pd/NGꢀAWꢀ2.2 and Pd/NGꢀUreaꢀ3.7,
Pd/NG-Urea
Pd/NG-AW
40
30
20
1
2
3
4
2
3
4
5
6
N-adding amount (mmol)
Fig. 6 The conversion of cyclohexene over Pd/NGꢀAW and Pd/NGꢀUrea
with different Nꢀadding amount. Reaction conditions: 0.01 g catalyst, 0.5
55 mL cyclohexene, 10 mL ethanol, 30 °C, 1.0 MPa, 1 h. ★ stands for the
conversion of cyclohexene at 20 °C.
30000
25000
20000
15000
10000
5000
0
hydrogenation reaction was carried out at 20 °C. Obviously,
1
2
3
4
5
Reaction cycle
Pd/NGꢀUreaꢀ3.7 showed a superior conversion of 83.7% to that
of 27.5% for Pd/NGꢀAWꢀ2.2.
3.2.3 Catalytic recycling
45 The catalytic recycling of Pd/NGꢀUreaꢀ3.7 was carried out at 30
Fig. 7 Turnover frequency in different catalytic cycles of Pd/NGꢀUreaꢀ3.7
in the hydrogenation of cyclohexene. Reaction conditions: 0.01 g catalyst,
60 2 mL cyclohexene, 10 mL ethanol, 30 °C, 1.0 MPa, 1 h.
°
C and 1.0 MPa, shown in Fig. 7. It can be seen that Pd/NGꢀ
3.2.4 Substrate expansion
The hydrogenations of different olefins over Pd/NGꢀUreaꢀ3.7
were performed at the same reaction conditions, and the results
Ureaꢀ3.7 can be reused for at least five times with no obvious
reduction of the catalytic activity. The TOFs were always over
26000 mol·(molPd·h)−1, indicating good stability for the catalyst
6
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C5RA12243K
bState Key Laboratory of Materials-oriented Chemical Engineering,
College of Chemistry and Chemical Engineering, Nanjing University of
50 Technology, Nanjing 210009, China
were showed in Table 4. Generally, aliphatic olefins including 1ꢀ
hexene and 1ꢀheptene had much low conversions, while styrene
had the highest conversion. The transformation rates of
cycloolefins (cyclopentene and cyclohexene) were in the middle
level. 36602 mol·(molPd·h)−1 of TOF was obtained in the
hydrogenation of styrene, more than three times to those of the
hydrogenation of aliphatic olefins. The ultrahigh TOF is possibly
attributed to the πꢀelectron interaction between graphene and the
benzene ring of styrene, which promotes the adsorption of styrene
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Ureaꢀ3.7 catalyst.
TOF
Reactant
Conversion (%) Selectivity (%)
(mol·(molPd·h)−1)
11144
1ꢀhexene
1ꢀheptene
cyclopentene
cyclohexene
styrene
23.9
20.1
52.6
58.9
78.5
100
100
100
100
100
9372
24526
27463
36602
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Catalytic Materials and Technology (No. BM2012110), the State
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Notes and references
a Address, Jiangsu Key Laboratory of Advanced Catalytic Materials and
Technology, College of Chemistry and Chemical Engineering, Changzhou
45 University, Gehu Road 1, Changzhou, Jiangsu 213164, PR China. Fax:
+86-519-86330135; Tel: +86-519-86330135;
115
E-mail: pingliu02@163.com (Ping Liu); liyxluck@163.com (Yong-Xin Li)
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