Effect of carbon nanosheets with different graphitization degrees as a support of noble metals on selective hydrogenation of cinnamaldehyde

Article abstract of DOI:10.1039/c6ra17979g

In this work, Pt and Pd catalysts supported on carbon nanosheets (CNS) with different graphitization degrees were prepared via a simple borohydride reduction method. The effect of graphitization degree on the catalytic activity of Pt/CNS and Pd/CNS was investigated to obtain the optimal activity. The prepared catalysts were well characterized by X-ray diffraction (XRD), Raman, transmission electron microscopy (TEM) and X-ray photoemission spectroscopy (XPS). The activity and selectivity of Pt/CNS and Pd/CNS catalysts increases with the rise of the graphitization degree of CNS. A high graphitization degree of the CNS support can enhance the transfer of electrons from the CNS support to Pt and Pd nanoparticles, leading to the increase in the surface Pt⁰ and Pd⁰ content. The more surface Pt⁰ and Pd⁰ content the catalyst has, the higher selectivity to COL and CALD it exhibits, respectively. The good graphitization degree of the CNS support is an advantage of the strong π-π interactions between CAL and the surface sp²-bonded of CNS support. Thus, the different catalytic activity and selectivity over the Pt/CNS and Pd/CNS catalysts are attributed to the surface Pt⁰ and Pd⁰ content and the adsorption capacity for CAL on the catalyst surface that are closely related to the graphitization degree of CNS.

Full text of DOI:10.1039/c6ra17979g

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Effect of carbon nanosheets with different
graphitization degrees as a support of noble metals
Cite this: RSC Adv., 2016, 6, 98356
on selective hydrogenation of cinnamaldehyde†
*
*
Qing Han, Yunfei Liu, Dong Wang, Fulong Yuan, Xiaoyu Niu and Yujun Zhu
In this work, Pt and Pd catalysts supported on carbon nanosheets (CNS) with different graphitization
degrees were prepared via a simple borohydride reduction method. The effect of graphitization degree
on the catalytic activity of Pt/CNS and Pd/CNS was investigated to obtain the optimal activity. The
prepared catalysts were well characterized by X-ray diffraction (XRD), Raman, transmission electron
microscopy (TEM) and X-ray photoemission spectroscopy (XPS). The activity and selectivity of Pt/CNS
and Pd/CNS catalysts increases with the rise of the graphitization degree of CNS. A high graphitization
degree of the CNS support can enhance the transfer of electrons from the CNS support to Pt and Pd
nanoparticles, leading to the increase in the surface Pt0 and Pd0 content. The more surface Pt0 and Pd0
content the catalyst has, the higher selectivity to COL and CALD it exhibits, respectively. The good
graphitization degree of the CNS support is an advantage of the strong p–p interactions between CAL
and the surface sp2-bonded of CNS support. Thus, the different catalytic activity and selectivity over the
Pt/CNS and Pd/CNS catalysts are attributed to the surface Pt0 and Pd0 content and the adsorption
capacity for CAL on the catalyst surface that are closely related to the graphitization degree of CNS.
Received 14th July 2016
Accepted 8th October 2016
DOI: 10.1039/c6ra17979g
www.rsc.org/advances
applications. To this end, heterogeneous catalysts have attrac-
ted lots of interest because they are environmental friendly and
Introduction
easy to be retrieved. Up to date, both noble metal (e.g., Pt, Pd, Ir,
and Ru) and non-noble metal (e.g., Fe, Co, Ni and Cu) nano-
particles (NPs) have been proved as highly efficient catalysts for
selectively producing COL and HALD.17–26 Generally, Pt and Pd
nanoparticles are considered as active metals for the hydroge-
nation of C]O and C]C bonds of CAL, respectively.20,23
Nevertheless, these NPs still have some drawbacks such as
spontaneous aggregation due to the high surface energy,
limited adsorption ability resulted from low surface area and
small pore volume, and readily contaminated catalytic active
sites.27–30 Therefore, anchoring nanoparticles on the supports
With the rapid development of catalyst science and technology,
controlling reaction selectivity has becoming more and more
signicant.1–3 As a typical example, selective hydrogenation of
a,b-unsaturated aldehydes is a common interest point both in
industrial and academic circles.4–6 For example, cinnamalde-
hyde (CAL) hydrogenation can produce cinnamal alcohol (COL),
hydrocinnamaldehyde (HALD), hydrocinnamal alcohol (HALC),
and so on (Scheme 1).7–11 Among them, COL is generally
considered to be the most challenging one to achieve. Also, it is
an important type of organic ingredient for chemical synthesis,
such as perfumes, cosmetics, medicines, and fungicides.12,13
The obtained COL still remains an arduous task because the
C]C bond reduction is more favorable than that of the C]O
functional group, thermodynamically. Therefore, high active
catalysts for selective production are urgently desired.
Recently, plenty of homogeneous catalysts have been re-
ported for selectively catalytic hydrogenation of CAL.14–16
However, the difficulty to retrieve the dissolved metal ions from
the chemical reaction vessel greatly hinders their widespread
Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University),
Ministry of Education, School of Chemistry and Materials, Heilongjiang University,
Harbin, 150080 P. R. China. E-mail: yujunzhu@hlju.edu.cn; niuxiaoyu2000@126.
com; Fax: +86-451-86609650; Tel: +86-451-86609650
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra17979g
Scheme 1 Reaction pathways for cinnamaldehyde hydrogenation.
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has been regarded to be a reasonable and reliable way to over- transferred to an ice bath (2–4 ꢀC) and stirred vigorously for 30
come the above mentioned difficulties.31–35
min. In addition, the slurry pH was adjusted to 8.0 using NaOH
Various reducible metal oxides such as CeO2,36,37 MnO2,38 (0.1 mol Lꢁ1) solution. Aerwards, an excess of NaBH4 (0.2
TiO2 (ref. 39 and 40) and ZnO41 have been used as the catalyst mol Lꢁ1) solution was added to the mixture, and stirred for 3 h.
supports and found to be able to improve the selectivity to COL The mixture was statically placed in an ice bath for 10 h. Finally,
and HALD productions. Besides metal oxides, carbon material 3.5 wt% Pt/CNS catalysts were obtained aer ltrating, washing
also has been demonstrated as a promising support for metal with deionized water and ethanol until no chloride ions detec-
NPs towards hydrogenation of unsaturated aldehyde.42–47 For ted and drying at 60 ꢀC for 10 h. The 3.5 wt% Pd/CNS catalysts
example, Shi et al. found that reduced graphene oxide sup- were prepared by following the same method. PdCl2 aqueous
ported Pt catalysts exhibited excellent catalytic performances in solution (5.00 g, Pd content: 3.50 mg gꢁ1) was added into the
terms of selectivity for the hydrogenation of unsaturated alde- mixture of CNS (0.500 g) support and 50 mL of deionized water.
hydes.42 In our previous report, the activity and selectivity may
be related with the strong p–p interactions between cinna-
maldehyde and graphene.43 In addition, we also found the
Catalyst characterization
property of supports, addition of MoN and WN can inuence X-ray diffraction (XRD) patterns were obtained with a Rigaku
˚
the dispersion, content and valence state of the surface Pt and D/max-IIIB diffractometer by using Cu-Ka (l ¼ 1.5406 A) radia-
Pd species, resulting to the change of the activity and selectivity tion. The accelerating voltage and applied current were 40 kV
in hydrogenation of CAL.16,25 So far, there are very few reports and 20 mA, respectively. Raman spectra were recorded with
about the effect of carbon support nature on the catalytic a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm.
performance for COL and HALD productions.
The laser beam was focused on the sample with a 50ꢂ objective.
In this work, a series of carbon nanosheets (CNS) with Transmission electron microscopy (TEM) was carried out with
different graphitization degree supported Pt and Pd catalysts a JOEL model JEM-2100 electron microscope working at an
were prepared through in situ self-generating template and acceleration voltage of 200 kV. The samples for TEM were sus-
post-loading two processes. The graphitization degree of CNS pended in ethanol and supported onto a holey carbon lm on
supports can be well controlled by the amount of growth cata- a Cu grid. X-ray photoemission spectroscopy (XPS) was carried
lysts (Fe species). The effect of CNS supports with different out on a Kratos-AXIS ULTRA DLD with an Al-Ka radiation
graphitization degree on catalytic performances of the Pt/CNS source. Scanning electron microscopy (SEM) was measured on
and Pd/CNS catalysts have been carefully studied for the selec- a Hitachi S-4800 eld emission electron microscope operating
tive hydrogenation of CAL.
at 20 kV. Surface area was measured at 77 K with a Micro-
meritics Tristar II 3020 analyzer. The sample was degassed
under vacuum at 423 K for 4 h prior to measurements. The
Brunauer–Emmett–Teller (BET) method was utilized to calcu-
late the specic surface areas using adsorption data in a relative
Experimental
Material preparation
Synthesis of CNS support with different graphitization pressure range from 0.05 to 0.25. The content of platinum and
degree. As reported previously, the CNS supports were fabricated palladium in the catalysts was determined by inductively
by an in situ self-generating template route.48 In a typical coupled plasma-optical emission spectrometry (ICP-OES),
synthesis, polyacrylic weak-acid cation-exchanged resin (PWAC) which was performed by a Perkin Elmer Optima 7000 DV
was added to aqueous FeCl2$4H2O or FeCl3$6H2O. Next, the analyzer. Before the measurement, each sample was dissolved
solution was stirred for a certain time under nitrogen protection. in a diluted HF and chloroazotic acid solution.
Then, the solid was collected by centrifugation and washed with
deionized water for two times. Finally, the solid was carbonized
at 1000 ꢀC for 1 h under N2. In order to remove the Fe species, the
Catalytic performance evaluation
obtained product was treated with 10% hydrochloric acid for 8 h. Catalytic performances of catalysts for the hydrogenation of
The solids were separated by centrifuge and washed several times CAL were carried out in a 100 mL autoclave. In brief, 50 mg of
with deionized water, and then dried in vacuum oven at 80 ꢀC for catalysts were immersed into 15 mL of isopropanol. The trace of
6 h. Five CNS supports were obtained controlled by adjusting the dissolved oxygen was removed by ushing with nitrogen and
reaction time in aqueous FeCl2$4H2O or FeCl3$6H2O solution, hydrogen at 5 bar for 3 times, respectively. The temperature was
denoted as CNS-1 (FeCl3$6H2O, 3 h), CNS-2 (FeCl2$4H2O, 1.5 h), raised to 110 ꢀC under 10 bar of hydrogen for 1 h to re-activate
CNS-3 (FeCl2$4H2O, 3 h), CNS-4 (FeCl2$4H2O, 6 h), CNS-5 the catalysts. A mixture of CAL (1.0 g) and isopropanol (15 mL)
(FeCl2$4H2O, 12 h), respectively.
was added into the autoclave. Aer ushing with nitrogen at 5
Preparation of 3.5 wt% Pt/CNS catalysts. The 3.5 wt% Pt/CNS bar for 3 times and subsequently with hydrogen at 5 bar for 3
catalysts were obtained through a simple borohydride reduction times, the reaction was allowed to proceed at 60 ꢀC under 10 bar
method. CNS (0.500 g) support was added to 50 mL of deionized of hydrogen for 2 h. The products were analyzed on a gas
water and subjected to ultrasonic treatment for 30 min. Then, chromatography (Agilent 7820A) equipped with an FID detector
H2PtCl6$6H2O aqueous solution (4.65 g, Pt content: 3.76 and an HP-5 capillary column (30 m ꢂ 0.32 mm ꢂ 0.25 mm).
mg gꢁ1) was added into the mixture. The resultant solution was
The turnover frequency (TOF) for the catalysts was calculated
further sonicated for 30 min. Subsequently, the slurry was by using eqn (1) at a CAL conversion.49
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(220) of the face-centered cubic Pt nanoparticles, respectively
(Fig. 1).52 The averaged crystal sizes of Pt nanoparticles calcu-
lated by full width at half maximum of Pt (111) reection based
on Scherrer's equation53 are 5.2, 5.4, 5.0, 5.2 and 5.0 nm for Pt/
CNS-1, Pt/CNS-2, Pt/CNS-3, Pt/CNS-4 and Pt/CNS-5 samples,
respectively (Table 1). It conrms that the particle size of Pt is
same for ve samples. The Pt loading was about 3.5 wt% ꢄ 0.2%
for the ve Pt/CNS samples based on ICP-OES. These catalysts
possess similar BET surface area of 56.5–60.1 m2 gꢁ1. The metal
dispersion was estimated based on the mean size of Pt particles
(equal to 1.13/mean size) assuming a cuboctahedral shape of
the metal particles were 21.7%, 20.9%, 22.6%, 21.7% and 22.6%
for Pt/CNS-1, Pt/CNS-2, Pt/CNS-3, Pt/CNS-4 and Pt/CNS-5,
respectively (Table 1).51,54
nðCALÞ ꢂ conv:%
TOF ¼
(1)
½mðcatÞyðPtÞ=MðPtÞꢃ ꢂ DðPtÞ ꢂ t
where n(CAL), m(cat), y(Pt) and M(Pt) represent the mole of CAL,
the catalyst weight, Pt loading level, and molar mass, respec-
tively, while conv.% and t are the conversion of CAL and the
reaction time, respectively. D(Pt) is the dispersion of surface Pt
sites and is estimated from the mean particle size of Pt.50,51
Results and discussion
In order to prepare carbon nanosheets (CNS) support with
different graphitization degree, Fe species (Fe2+ and Fe3+) was
chosen as graphitization catalyst to form the graphene-like
structure.48 XRD patterns of the obtained CNS supports
exhibit four peaks at 27.0ꢀ (002), 42.5ꢀ (100), 54.6ꢀ (004) and
77.4ꢀ (110) which are characteristic facets of graphite (Fig. S1†).
From CNS-1 to CNS-5, the graphitization degree of CNS support
increases according to the IG/ID values of the Raman spectra for
ve CNS supports (Fig. S2†). SEM images of ve CNS supports
show carbon nanosheets morphology with awed and not at
(Fig. S3†). Combined above characterizations, carbon nano-
sheets with different graphitization degree were successfully
prepared as support. No Fe species was observed by XPS of
Pt/CNS and Pd/CNS catalysts (Fig. S4†), suggesting Fe was
removed by 10% hydrochloric acid in the preparation process of
CNS.
Raman spectroscopy
Raman is one of useful tools to provide structure details of
carbon materials, such as disorder and defect structure. In the
Raman spectra of the as-obtained Pt/CNS catalysts (Fig. 2), two
prominent peaks at 1368 and 1590 cmꢁ1 were observed, which
were well assigned to the breathing mode of k-point phonons of
A
1g symmetry (D band) and E2g phonons of sp2 carbon atoms (G
band), respectively.55 It is well known that a high value in the
Table
1 Physicochemical property of the as-obtained Pt/CNS
catalysts
XRD analysis
a
SBET
Pt crystal sizeb
(nm)
Pt dispersionc
(%)
d
Catalyst
(m2 gꢁ1
)
IG/ID
XRD patterns of the Pt/CNS catalysts exhibit a sharp and strong
diffraction peak at 27.0ꢀ, which corresponds to the facet (002) of
graphite (Fig. 1).48 It suggests that the CNS supports have an
ordered structure and high graphitization degree. More
importantly, the peak intensity increased with treatment time
in FeCl2$4H2O, indicating that the graphitization degree of CNS
supports can be well controlled. Other diffraction peaks
observed in the patterns of all catalysts at 2q of 39.7, 46.4, and
67.5ꢀ can be assigned to the crystalline planes (111), (200), and
Pt/CNS-1
Pt/CNS-2
Pt/CNS-3
Pt/CNS-4
Pt/CNS-5
60.1
59.4
59.7
58.6
56.5
5.2
5.4
5.0
5.2
5.0
21.7
20.9
22.6
21.7
22.6
1.9
3.5
4.3
4.9
6.7
BET surface area. b Calculated from Pt(111) in XRD patterns based on
Scherrer's equation. Estimated from the mean particle size of Pt.
Calculated from the Raman spectra according to the intensities of D
a
c
d
and G peaks.
Fig. 1 XRD patterns of Pt/CNS-1, Pt/CNS-2, Pt/CNS-3, Pt/CNS-4 and Fig. 2 Raman spectra of Pt/CNS-1, Pt/CNS-2, Pt/CNS-3, Pt/CNS-4
Pt/CNS-5 catalysts.
and Pt/CNS-5 catalysts.
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intensity ratio of G to D (IG/ID) indicates a better degree of catalysts (Fig. 3b, c, e, f, h and i) disclose that high crystalline
graphitization in the carbon materials. The IG/ID values for Pt/ Pt NPs closely contact with CNS support. The thin layer edges of
CNS-1, Pt/CNS-2, Pt/CNS-3, Pt/CNS-4 and Pt/CNS-5 catalysts carbon nanosheets were observed clearly (Fig. 3b, e and h). The
are calculated to be 1.9, 3.5, 4.3, 4.9, and 6.7, respectively. It two kind of well-dened lattice fringes can be clearly observed in
suggests that the graphitization degree of CNS support Fig. 3c, f and i. The measured lattice spacing value of Pt NPs and
increases from Pt/CNS-1 to Pt/CNS-5, which is in good agree- CNS support is 0.223–0.227 and 0.352–0.379 nm (Fig. 3c, f and i),
ment with the XRD results. Raman and XRD results conrm the which can be well assigned to the planes (111) of the face-
obtained Pt/CNS catalysts with different graphitization degree centered cubic Pt and the (002) planes of graphite, respectively.
for CNS supports.
This explains that the Pt/CNS catalysts have a graphitic structure.
In addition, with the increase in graphitization degree of CNS
supports, the layer edges of carbon nanosheets and the lattice
spacing were much clearer. From Fig. 3, it can be seen that the Pt/
CNS-5 catalyst have a better graphitic structure than Pt/CNS-1
and Pt/CNS-3. These analyses prove that the Pt/CNS materials
with graphitic structures were successfully synthesized. The
corresponding particle size distributions (Fig. 3j–l) reveal that the
average particle sizes of Pt NPs are 4.9, 4.8, and 5.4 nm for Pt/
TEM analysis
TEM analysis provided more detailed information about the
particle dispersion and particle size of the Pt/CNS catalysts. TEM
images (Fig. 3a, d and g) of Pt/CNS-1, Pt/CNS-3, and Pt/CNS-5
catalysts clearly show that Pt NPs are well dispersed on CNS
support. HRTEM images of Pt/CNS-1, Pt/CNS-3 and Pt/CNS-5
Fig. 3 TEM images, HRTEM images and corresponding particle size distribution plots of (a–c, j) Pt/CNS-1, (d–f, k) Pt/CNS-3 and (g–i, l) Pt/CNS-5
catalysts. The average diameters obtained by measuring about 100 particles for each catalyst.
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CNS-1, Pt/CNS-3 and Pt/CNS-5, respectively. The results are well CNS. In terms of quantitation of the composition of the Pt
consistent with the calculated results based on Scherrer's equa- nanoparticles, the content of the surface Pt0 species was esti-
tion from XRD patterns.
mated to be 50.8%, 54.0%, 54.8%, 57.8% and 67.2% for Pt/CNS-
1, Pt/CNS-2, Pt/CNS-3, Pt/CNS-4 and Pt/CNS-5, respectively (Table
2), which demonstrates the amount of the Pt0 species increases
from Pt/CNS-1 to Pt/CNS-5. However, the changed degree in the
surface Pt0 content is slight Pt/CNS-1 to Pt/CNS-4, suggesting they
have similar Pt0 content.
X-ray photoelectron spectroscopy
The C 1s spectra of XPS have been analyzed and shown in Fig. 4.
From the C 1s spectra of ve samples, four different peaks
centered at about 284.5, 285.7, 287.9 and 289.7 eV were
observed, corresponding to C–C, C–O, C]O and O–C]O
groups, respectively (Fig. 5).45,46 Based on the integration of
corresponding peaks, the percentage of sp2 hybridized C–C
bonds were 32.4%, 42.4%, 44.1%, 46.2% and 49.7%, respec-
tively. It is in good agreement with the Raman results. It also
proves that graphitization degree increase for the supports from
CNS-1 to CNS-5.
Catalytic performances
The Pt/CNS catalysts were applied for the selective hydrogena-
tion of CAL. The major products obtained by the hydrogenation
XPS was used to study the surface Pt species of Pt/CNS cata-
lysts (Fig. 5). It is clearly found that Pt 4f signal consists of three
chemically different species from the spin-split obtained by
tting Gaussian peaks aer Shirley-background subtraction. The
most intense pair of peaks (71.4 and 74.7 eV), the second doublet
(72.6 and 75.9 eV) and the third doublet (74.9 and 78.0 eV) can be
assigned to metallic Pt0, Pt2+ in the form of PtO and Pt(OH)2, and
Pt4+ possibly in the form of PtO2, respectively,56–59 as summarized
in Table 2. The binding energy of metallic Pt0 is high due to Pt–C
strong interaction, resulting in a fast electron transfer from Pt to
Fig. 5 Pt 4f high-resolution XPS spectra of Pt/CNS-1, Pt/CNS-2, Pt/
CNS-3, Pt/CNS-4, and Pt/CNS-5 catalysts.
Table 2 XPS data of distributions of functional groups
Pt species
C species contenta (%)
contenta (%)
Catalyst
C]C
C–O
C]O
O–C]O
Pt0
Pt2+
Pt4+
Pt/CNS-1
Pt/CNS-2
Pt/CNS-3
Pt/CNS-4
Pt/CNS-5
32.4
42.4
44.1
46.2
49.7
41.4
30.2
30.5
29.5
22.9
6.8
3.6
1.1
2.1
3.9
19.4
23.8
24.3
22.2
23.5
50.8
54.0
54.8
57.8
67.2
35.8
39.4
32.1
36.5
23.2
13.4
6.6
13.1
5.7
9.6
Fig. 4 C 1s high-resolution XPS spectra of Pt/CNS-1, Pt/CNS-2, Pt/
CNS-3, Pt/CNS-4, and Pt/CNS-5 catalysts.
a
Calculated from the XPS results.
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of CAL involve COL, HALD and HALC (Table 3). In order to clear
disclose the effect of CNS supports with different graphitization
degree for Pt/CNS catalysts on the conversion and selectivity in
the hydrogenation reaction of CAL, the evaluated experiments
of catalytic performance were carried out under mild reaction
conditions (10 bar H2 at 60 ꢀC for 2 h). The conversion of CAL is
14.4%, 16.1%, 24.3%, 43.5% and 58.7% over Pt/CNS-1, Pt/CNS-
2, Pt/CNS-3, Pt/CNS-4, and Pt/CNS-5, respectively. The turnover
frequency (TOF) were 7.76, 9.01, 12.6, 23.4 and 30.4 sꢁ1 for Pt/
CNS-1, Pt/CNS-2, Pt/CNS-3, Pt/CNS-4, and Pt/CNS-5, respec-
tively. Meanwhile, the selectivity to COL also increases from Pt/
CNS-1 (16.7%) to Pt/CNS-5 (67.3%), in which the highest
selectivity towards COL was obtained over the Pt/CNS-5 catalyst.
When the reaction was performed at 80 ꢀC under 10 bar H2 for 4
h, CAL conversion and selectivity to COL can reach 87.5% and
90.5% over Pt/CNS-5, respectively.
Fig. 6 Cinnamaldehyde conversion versus reaction time in cinna-
maldehyde hydrogenation over Pt/CNS-1, Pt/CNS-2, Pt/CNS-3,
Pt/CNS-4, and Pt/CNS-5 catalysts (reaction conditions: 50 mg 3.5 wt%
Pt/CNTs, 1.00 g CAL, 30 mL isopropanol, 10 bar H2, 60 ꢀC, 2 h).
A serial of experiments about the traces of conversion and
selectivity as a function of reaction time were also carried out
over the Pt/CNS catalysts and the results are display in Fig. 6 and
7. At initial 20 min, the conversion of CAL was less than 8% for
all Pt/CNS catalyst. Aer a reaction time of 60 min, the
conversion of CAL increased to 8.9%, 9.1%, 10.3%, 14.6% and
33.1% over Pt/CNS-1, Pt/CNS-2, Pt/CNS-3, Pt/CNS-4 and Pt/CNS-
5, respectively. And the selectivity to COL was 17.0%, 24.6%,
42.3%, 60.7% and 69.4% over Pt/CNS-1, Pt/CNS-2, Pt/CNS-3, Pt/
CNS-4, and Pt/CNS-5, respectively. On the whole, the conversion
of CAL increases gradually with time, and the selectivity to COL
slightly decreases in 120 min.
The recycling experiments were performed over Pt/CNS-5
based on its better catalytic activity. The catalyst was reused
directly without any treatment aer precipitated and separated
from the reaction solution. In every run, the activity exhibited
a slight decrease compared with the results last run. Aer four
used cycles, the conversion of CAL declined from 58.7% to 45%,
and the selectivity to COL decreased from 67.3% to 46.5%
(Fig. 8). According our previous studies,16,43 the deactivation
reasons can be attributed to the aggregation of Pt particles, the
reduced amount of Pt0 species on the surface and the formed
side product with large molecular weights adsorbed on the
active sites.
Fig. 7 Selectivity as a function of reaction time of Pt/CNS-1, Pt/CNS-2,
Pt/CNS-3, Pt/CNS-4, and Pt/CNS-5 catalysts (reaction conditions:
50 mg 3.5 wt% Pt/CNTs, 1.00 g CAL, 30 mL isopropanol, 10 bar H2,
60 ꢀC, 2 h).
Table 3 Catalytic performances of Pt/CNS catalysts
Selectivity (%)
Conversion
Catalysts
(%)
TOFc (sꢁ1
)
HALD HALC COL Othersd
Pt/CNS-1a 14.4
Pt/CNS-2a 16.1
Pt/CNS-3a 24.3
Pt/CNS-4a 43.5
Pt/CNS-5a 58.7
Pt/CNS-5b 87.5
7.76
9.01
12.6
23.4
30.4
22.7
1.0
1.2
1.5
3.9
2.9
1.3
2.7
2.5
1.6
2.0
2.2
1.8
16.7 79.6
21.9 74.4
41.6 55.3
58.9 35.2
67.3 27.6
90.5
6.4
a
b
Reaction conditions: under 10 bar H2 at 60 ꢀC for 2 h. Reaction
c
conditions: under 10 bar H2 at 80 ꢀC for 4 h. Calculated at CAL
conversion.
d
Includes 1-(3-propoxyprop-1-enyl)benzene, cinnamyl
formate, cinnamic acid, benzyl, cinnamate, 4,4-diphenylcyclohexa-1,5-
dienyl acetate, and other condensation products that could not be
identied by GC-MS because of their large molecular weights.
Fig. 8 Stability test of Pt/CNS-5 (reaction conditions: 50 mg 3.5 wt%
Pt/CNTs, 1.00 g CAL, 30 mL isopropanol, 10 bar H2, 60 ꢀC, 2 h).
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It is noticed that the order of conversion to CAL and selec- well graphitization degree of CNS support is advantage of the
tivity to COL is Pt/CNS-5 > Pt/CNS-4 > Pt/CNS-3 > Pt/CNS-2 > Pt/ strong p–p interactions between CAL and the surface sp2-
CNS-1. According to above characterization results, Pt/CNS bonded of CNS support, which results to the high conversion of
catalysts with similar Pt content and mean particle size values CAL.63 Moreover, the high p–p interactions can favor of the
of Pt were obtained by supporting Pt on CNS with different contact of C]O with Pt species due to CAL structure, which
graphitization degree. Thus, the effect of Pt content and its leads to the increase in selectivity to COL.41
mean particle size on the catalytic activity should be excluded
The best catalytic activity and selectivity over Pt/CNS-5 can be
for these Pt/CNS catalysts in this study. In our previous attributed to the highest graphitization degree of CNS-5
studies,16,43,60 the amount of surface Pt0 species was considered support. So it can see clearly that the graphitization degree of
as a key species in responsibility of the selectivity to COL. In carbon nanosheets support plays an important role on the
addition, Wang et al. found Pt2+ species played an important selective hydrogenation of CAL.14 Therefore, the different cata-
role on the selectivity to COL when the ratio of Pt2+ to Pt (Pt2+/Pt) lytic activity and selectivity over the Pt/CNS catalysts are attrib-
is more than 1.0.56 However, in our studies, the content of Pt0 uted to the surface Pt0 content and the adsorption capacity for
species is more than that of Pt2+ species, which the effect of Pt2+ CAL on the catalyst surface that are closely related to the
species on the activity and selectivity can be ignored. Generally, graphitization degree of CNS.
Pt4+ species can lead to form the undesired condensation
Generally, Pd is favor of hydrogenation of C]C in the
products due to its acidity. Thus, Pt0 species should play the selective hydrogenation of CAL.25 In order to conrm above
dominant role in selective hydrogenation of CAL. The more standpoint, we prepared a series of Pd NPs supported on
surface Pt0 content catalyst has, the higher selectivity to COL it different graphitized carbon nanosheets supports (Pd/CNS) to
exhibits. In general, the Pt0 content increases with graphitiza- study the effect of different graphitized CNS supports on the
tion degree of CNS support from Pt/CNS-1 to Pt/CNS-5 accord- selective hydrogenation of CAL. The results of selective hydro-
ing to the results of XPS (Fig. 5), XRD (Fig. 1) and Raman (Fig. 2). genation of CAL are displayed in Table 4 over Pd/CNS. The
The change of the amount of the Pt0 species among these Pt/ conversion of CAL for Pd/CNS-1, Pd/CNS-2, Pd/CNS-3, Pd/CNS-4
CNS catalysts can be ascribed to the difference in the interac- and Pd/CNS-5 was 24.4%, 37.4%, 42.4%, 56.8% and 59.1%,
tion between Pt and CNS support with different graphitization respectively, when the reaction was carried out under 10 bar H2
degree. A large amount of C–OH, COOH and C]O groups at 60 ꢀC for 2 h (Table 4). Meanwhile, HALD was main product
existing on the CNS with much lower graphitization degree and its selectivity also increased as the rise of graphitization
result to the transfer of more electrons from Pt to CNS support,61 degree of CNS supports. The Pd/CNS-5 catalyst showed the
which would lead to the formation of high oxidation state Pt highest selectivity (ꢅ65.3%) towards HALD among the ve
species (Pt2+ and Pt4+) and the decrease in the content of Pt0 catalysts. The turnover frequency (TOF) was 13.4, 21.6, 24.2,
species. The work by Guo et al. also conrmed it.62 They studied 32.1 and 32.8 sꢁ1 for Pd/CNS-1, Pd/CNS-2, Pd/CNS-3, Pd/CNS-4
the effect of the surface properties of the carbon nanotube and Pd/CNS-5, respectively.
(CNT) support on the hydrogenation results of CAL. Their
The similar mean particle size (ꢅ10 nm) of Pd was obtained
results showed the conversion and selectivity for hydrogenation for all Pd/CNS catalysts from XRD results (Fig. S5†). Hence, the
of CAL highly depended on the surface properties of the CNT effect of mean particle size of Pd on catalytic activity should be
support over Pt/CNT catalyst. Removal of oxygen-containing also excluded for these Pd/CNS catalysts. It has been found that
groups from CNT surfaces elevated both activity and selec- the Pd0 species plays a key role on the selectivity to HALD in
tivity, due to the suppressed side reactions catalyzed by acid and the selective hydrogenation of cinnamaldhyde.25 The amount of
enhanced electron transfer from CNT to Pt nanoparticles.62 In Pd0 species is 32.5%, 40.7%, 49.4%, 59.6% and 62.4% on
other word, with the increase in graphitization degree of the surface of Pd/CNS-1, Pd/CNS-2, Pd/CNS-3, Pd/CNS-4 and
support, fewer and fewer oxygen-containing groups on the Pd/CNS-5, respectively (Table S1 and Fig. S6†), which
support surface can enhance the transfer of electron from CNS
support to Pt. The enriched electron density surrounding the Pt
nanoparticles reduces the probability of C]C bond coordi-
nated to Pt active sites due to the electronic repulsion, thereby
increases the selectivity toward COL.18
Table 4 The catalytic performances of Pd/CNS catalysts
Selectivity (%)
The highest selectivity and conversion can be attributed to
Catalysts Conversion (%) TOFa (sꢁ1
)
HALD HALC COL Othersb
the largest amount of Pt0 species on the surface the Pt/CNS-5
sample.12,58 It can be seen that the selectivity to COL increased
from 16.7% to 58.9% with graphitization degree of CNS support
for Pt/CNS-1, Pt/CNS-2, Pt/CNS-3 and Pt/CNS-4. However, the Pt0
content increases slightly about 7% from Pt/CNS-1 to Pt/CNS-4.
Therefore, we think that the difference in conversion to CAL and
selectivity to COL for the Pt/CNS catalysts should be also relate
to the adsorbed property of different CNS supports for CAL. The
better graphitized degree carbon nanosheets support is, the
higher adsorption capacity for CAL it shows on its surface. The
Pd/CNS-1 24.4
Pd/CNS-2 37.4
Pd/CNS-3 42.4
Pd/CNS-4 56.8
Pd/CNS-5 59.1
13.4
21.6
24.2
32.1
32.8
36.4
48.7
53.4
58.7
65.3
14.3
15.7
17.9
21.0
9.8
3.1 46.2
3.9 31.7
2.7 26.0
0
20.3
3.9 21.0
a
b
Calculated at CAL conversion. Includes 1-(3-propoxyprop-1-enyl)
benzene, cinnamyl formate, cinnamic acid, benzyl, cinnamate, 4,4-
diphenylcyclohexa-1,5-dienyl acetate, and other condensation
products that could not be identied by GC-MS because of their large
molecular weights.
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demonstrate that high graphitized degree of CNS is advantage
of formation of the surface Pd0 species due to enhancing the
electron transfer from CNS support to Pd. The Pd/CNS-5 catalyst
shows the highest conversion (ꢅ59.1%) and the highest selec-
tivity (ꢅ65.3%) towards HALD. And the other main reason is
that the different graphitized carbon nanosheets supports play
other roles on the conversion of CAL. The better graphitized
carbon nanosheets support is, the more adsorption of CAL is on
its surface. Although the strong p–p interactions is also
advantage of the contact of C]O with Pd species, the hydro-
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In summary, different graphitized carbon nanosheets materials
were investigated as a support for Pt and Pd catalysts for the
selective hydrogenation of CAL. The graphitization degree of
CNS supports can be well controlled by the amount of growth
catalysts (Fe species). The Pt/CNS and Pd/CNS catalysts have
nanosheets structures, highly dispersed Pt nanoparticles with
a particle size of ꢅ5 nm and Pd nanoparticles with a particle
size of ꢅ10 nm, respectively. The activity and selectivity of Pt/
CNS and Pd/CNS catalysts increases as the rise of graphitiza-
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the Pt/CNS and Pd/CNS catalysts are attributed to the surface Pt0
and Pd0 content and the adsorption capacity for CAL on the
catalyst surface that are closely related to the graphitization
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This work was supported by Natural Sciences Fund of Hei-
longjiang Province (B2015009), the Innovative Research Project
of Key Laboratory of Functional Inorganic Material Chemistry
(Heilongjiang University), Ministry of Education (2015), the
Scientic Research Foundation for the Returned Overseas
Chinese Scholars, State Education Ministry (2013-1792) and
Ministry of Human Resources and Social Security (2013-277).
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