Selective hydrogenation of cinnamaldehyde over Pt nanoparticles deposited on reduced graphene oxide

Article abstract of DOI:10.1039/c3ra44962a

The selective hydrogenation of cinnamaldehyde was investigated with Pt nanoparticles deposited on reduced graphene oxide (Pt/RGO). Compared with carbon nanotubes (CNTs) or activated carbon (AC) as support, Pt/RGO showed the best activity and selectivity for the hydrogenation of CO bond.

Full text of DOI:10.1039/c3ra44962a

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Selective hydrogenation of cinnamaldehyde over Pt
nanoparticles deposited on reduced graphene
oxide†
Cite this: RSC Adv., 2014, 4, 1874
Received 9th September 2013
Accepted 21st November 2013
*
Zhuohua Sun, Zeming Rong, Yong Wang, Yan Xia, Wenqiang Du and Yue Wang
DOI: 10.1039/c3ra44962a
www.rsc.org/advances
The selective hydrogenation of cinnamaldehyde was investigated with more reactive than the C]O one. The key factor controlling the
Pt nanoparticles deposited on reduced graphene oxide (Pt/RGO). activity and selectivity remains unclear, although it is well
Compared with carbon nanotubes (CNTs) or activated carbon (AC) as known as a general rule that transition metals12 and support
support, Pt/RGO showed the best activity and selectivity for the morphology13 might affect the process.
hydrogenation of C]O bond.
Serp et al. used carbon nanotubes (CNTs) and activated
carbon (AC) for the selective hydrogenation of cinnamaldehyde
to study the support effects.14 CNTs support with mesoporous
structure made mass transfer limitations less signicant and
gave better activities than AC.
In this study, graphene was employed as support for the
selective hydrogenation of cinnamaldehyde due to its unique
conguration of a 2-dimension (2-D) plane. Compared with Pt
based CNTs and AC, Pt/RGO showed the best activity and
selectivity for the hydrogenation of C]O bonds, which
provided a facile, economical, and environmentally benign
alternative for the synthesis of unsaturated alcohols.
The colloid of graphene oxide was prepared by improved
Hummers' method15 and subsequently treated with sonication
and centrifugation. Then GO sample and H2PtCl6 were mixed in
a solution containing deionized water and ethylene glycol (EG)
with continuous stirring at room temperature for 2 h. The above
mixture was treated in an ultrasonic bath (140 W) for 2 h to
ensure that most GO were fully exfoliated, and then it was
heated to 393 K in a stainless steel autoclave with Teon inner
layer for 24 h (see ESI†). Pt nanoparticles deposited on multi-
walled carbon nanotubes and activated carbon were prepared
by the same procedures and were denoted as Pt/CNTs and
Pt/AC, respectively. EG was selected as a reductant since it could
well control the mean particle size of the metal. In addition, as
an intercalating agent like pyridine,16 it is favorable to inhibit
the re-stacking of GO during the reaction process.
Raman spectroscopy was employed to characterize carbo-
naceous materials for distinguishing ordered and disordered
crystal structures.17 Raman spectra of natural graphite, GO and
Pt/RGO were shown in Fig. 1a. G band is assigned to the E2g
phonons of sp2 C atoms. D band is assigned to a breathing
mode of k-point photons of A1g symmetry which corresponds
to the defects in the curved graphene sheet with staging
Graphene, a monolayer sheet of arrayed sp2 carbon atoms
densely packed in a honeycomb crystal lattice, has attracted
tremendous attention as the promising candidate for hetero-
geneous catalyst support1–5 owing to its large surface area
(theoretically, ca. 2630 m2 gꢀ1) and outstanding electronic
conductivity (2 ꢁ 105 cm2 Vꢀ1 Sꢀ1).6 Generally, the graphene for
loading metals or metal oxides was synthesized by the reduction
of graphene oxide (GO),7 which provided a variety of function-
alities (such as surface function groups), holes, carbon vacan-
cies and defects8 to anchor and accommodate a great amount of
metal nanoparticles with ne dispersion.
The graphene based catalysts have been explored with RGO
method, a suitable way for preparing graphene in large scale at a
relatively low cost. Nie et al.9 employed ethylene glycol (EG)
reduction method to produce Pt/RGO catalyst which showed
excellent reactivity in the hydrogenation of nitroarenes. Sheng
et al.10 prepared highly-dispersed ultrane Pt nanoparticles
supported on graphene sheets. Using this novel catalyst, various
unsaturated compounds were reduced with excellent yields
under mild conditions. However, all these work simply focused
on promoting activity rather than selectivity.
Selective hydrogenation of a,b-unsaturated aldehydes to the
corresponding unsaturated alcohols is important for both
fundamental science and applications.11 Accordingly, the
development of catalysts for this class of reactions is still chal-
lenging, since the C]C bond is normally thermodynamically
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, Dalian 116024, P. R. China. E-mail: zeming@dlut.edu.cn;
Tel: +86 411 84986242
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra44962a
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Furthermore, Pt/RGO was analysed by XPS and the results
were presented in Fig. 1c and d (XPS results for Pt/CNTs and Pt/
AC were presented in Fig. S1†). The asymmetric C1s spectrum
was deconvoluted into four peaks at 284.6, 286.6, 287.7 and
289.0 eV, which were assigned to the C element in C–C, C–O,
C]O and O–C]O group respectively.24 Based on the integra-
tion of corresponding peaks, the percentage of sp2 hybridized
C–C bonds was 77.0% (Table 2), which proved a good reduction
of GO.25 Fig. 1d showed Pt4f spectra for Pt/RGO. The most
intense doublet with binding energies of 71.3 eV (Pt4f7/2) and
74.5 eV (Pt4f5/2) were attributed to the metallic Pt. In contrast,
the peaks at 72.2 eV (Pt4f7/2) and 75.6 eV (Pt4f5/2) could be
assigned to Ptd+ that anchored with the C–O group in RGO.9
From the integration of different peaks it could be concluded
that 54.1% of Pt existed as metallic state and others existed as
oxidized state aer reduction. Among three supports, the
reduction degree for RGO was higher than CNTs and AC.
Fig. 2 illustrated the XRD patterns of graphite, GO and Pt-
based catalysts. For graphite sample, the sharp and strong
diffraction peak at 27ꢃ was assigned to (002) facet of graphite
and it disappeared aer oxidation. A strong diffraction peak
appeared at 10.5ꢃ which was the reection of the few-layer
graphene structure.21 However, the peak at 10.5ꢃ disappeared
once again aer reduction and was replaced by a weak and
broad peak between 17ꢃ and 30ꢃ which was assigned to the
characteristic structure of RGO. This change could be attributed
to the formation of “re-graphitized” carbon regions and
restacking due to the van der Waals attractive interactions. The
sharp diffraction peaks at 39.7ꢃ and 46.4ꢃ in Pt-based catalysts
typically belong to the (111) and (200) facets of Pt. Using
Fig. 1 (a) Raman spectra of GO, Pt/RGO and graphite (b) AFM image
and section analysis of GO (c) XPS spectra of C1s region for Pt/RGO (d)
XPS spectra of Pt4f region for Pt/RGO.
disorder.18 As a result, the natural graphite powder provided a
strong G band at ꢂ1587 cmꢀ1, but a weak D band at ꢂ1351
cmꢀ1. Aer being oxidated, exfoliated and then reduced, the
ordered crystal structure of graphite was broken. Therefore the
lattice defect obviously increased and the D band became
strong for GO and Pt/RGO. Table 1 summarized the Raman
band positions and the crystalline size of GO and Pt/RGO. The
intensity ratio of D band to G band (ID/IG) was even higher for
Pt/RGO than GO since the re-established graphene network
processed smaller average size.19 The ratio of the integral
intensities of the G and D bands is related to the in-plane
Table 2 XPS data of distributions of functional groups
crystallite size, which were calculated with the equation: La ¼
(2.4 ꢁ 10ꢀ10)llaser4(ID/IG)ꢀ1
.
The crystalline domain sizes for
20
Relative atomic percentage (%)
GO and Pt/RGO were calculated to be 19.8 nm and 15.0 nm,
respectively.
Sample
C]C
C–O
C]O
O–C]O
Pt
Ptd+
Fig. 1b displayed an AFM image of the GO sample, which
was prepared by depositing the corresponding dispersions on
new cleaved mica surfaces and drying at room temperature.
The lateral dimensions for the GO sheets ranged from 0.1 to
1 mm and the height was about 1.0 nm, which was typical for
one-atom-thick graphene oxide.21 The height was some thicker
than the theoretical value,22 which could be ascribed to indi-
vidual graphitic sheets bearing oxygen-containing groups on
both faces.23
GO
38.5
77.0
62.9
57.7
13.5
10.1
13.8
17.7
11.0
3.9
7.4
7.3
37.0
9.0
15.9
17.3
—
—
Pt/RGO
Pt/CNTs
Pt/AC
54.1
50.4
55.2
45.9
49.6
44.8
Table 1 Raman spectroscopy data and crystalline sizes of GO and Pt/
RGO
Band positions
(cmꢀ1
)
a
Sample
D
G
ID/IG
La (nm)
GO
Pt/RGO
1350
1345
1593
1584
0.97
1.28
19.8
15.0
a
Determined from the relative peak heights in Raman spectra.
Fig. 2 XRD patterns of graphite, GO, Pt/RGO, Pt/C and Pt/CNTs.
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only deposited on its outer surface due to the unopened
terminals. The graphene sheets provided typical planar struc-
ture and the position with deep color demonstrated a folded
region at edges. In addition, Pt nanoparticles deposited on
graphene with ne dispersion, indicating a strong metal-
support interactions.26
Some properties of the catalysts might dominate the selec-
tive hydrogenation of cinnamaldehyde, such as the sorts of
transition metal,12 particle size,27 types of support28 and the
preparation method29 et al. In this study, Pt-based catalysts were
controlled between 2.3 nm and 2.5 nm using the same prepa-
ration method. The performance of three catalyst supports was
investigated by the selective hydrogenation of cinnamaldehyde
and the results were summarized in Table 3. Clearly, Pt/RGO
showed best activity under the same reaction conditions and it
was 1.5 and 1.4 times higher than that of Pt/AC and Pt/CNTs
catalysts. Activated carbon (AC) has the largest surface area
(1028.7 m2 gꢀ1) but with the lowest activity. The numerous
microspores of AC might hinder the diffusion of the liquid
medium,14 so the hydrogenation reaction proceeded more
slowly. CNTs with mesoporous structure made mass transfer
limitations less signicant and gave almost same activity like
AC but with the least surface area (157.6 m2 gꢀ1). RGO possesses
unique 2D morphology and both surfaces of the sheet are
accessible for reactants, which obviously decreases the diffu-
sional limitations. So Pt/RGO showed best catalytic perfor-
mance for the hydrogenation of cinnamaldehyde.
Additionally, the selectivity with Pt/RGO to CALC was higher
than that of other two catalysts, which was presumably caused
by the support effects. Firstly, graphene had the least oxygen-
containing surface groups (Table 2) and outstanding electronic
conductivity,6 so the electron was easily transferred from the
support to the metal centre,30 which favored the back-bonding
interactions with p*C]O to a larger extent than pC* ]C and
increased CALC selectivity.31,32 On the other hand, the aromatic
rings prefer to orient in parallel with the plane of graphene
(Fig. 4) due to p–p interactions.33 The curvature effect forces the
aromatic rings far away from the surface of CNTs. Comparing
two adsorption modes in Fig. 4, the C]O bonds could be
selectively hydrogenated over Pt atom sites supported on RGO
than CNTs. For AC support, it is difficult to control the selec-
tivity due to the irregular surface and porous properties.
In order to clarify the hydrogenation process of cinna-
maldehyde over Pt/RGO catalyst, the component concentration
versus reaction time were monitored by GC and the results were
Fig. 3 TEM images of Pt/AC (a and b), Pt/CNTs (c and d), Pt/RGO (e
and f).
Scherrer's equation, the average crystalline sizes of Pt on RGO,
CNTs and AC supports were estimated to be about 3.4, 4.0 and
2ꢀ
3.6 nm, respectively. It clearly indicated that PtCl6
was
reduced by EG with the uniform size in different supports.
Fig. 3 showed the TEM images of three catalysts. Pt nano-
particles uniformly decorated on different supports with almost
same average size (2.3 nm for Pt/C, 2.5 nm for Pt/CNTs and 2.4
nm for Pt/RGO), which was slightly smaller than calculated
samples with XRD. Moreover, the images for different supports
were easily distinguished by the surface morphology. The active
carbon possessed numerous pores and Pt nanoparticles closely
packed on it with somewhat aggregation. For carbon nanotubes
support, many tubes intertwined together and Pt nanoparticles
Table 3 Hydrogenation of cinnamaldehyde in different catalystsab
Selectivity (%)
Catalyst
SBET (m2 gꢀ1
)
Conversion (%)
CALC
HALD
HALC
Others
Activity (mol-CALD/(mol-Pt h))c
Pt/C
Pt/CNTs
Pt/RGO
1028.7
157.6
276.1
84.2
87.2
89.6
30.9
48.3
69.6
27.0
20.8
9.2
35.2
26.2
17.6
6.8
4.7
3.6
310.9
322.0
456.4
a
b
Reaction conditions: cat. 0.02 g, CALD 0.5 g, isopropanol 10 ml, 313 K, 2 MPa, 2.5 h. CAL: cinnamaldehyde, CALC: cinnamyl alcohol, HALD:
hydrocinnamaldehyde, HALC: hydrocinnamyl alcohol. c Pt content were 2.9 wt%, 4.0 wt% and 4.0 wt% for Pt/RGO, Pt/CNTs and Pt/C respectively.
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Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (21206015), Specialized Research
Fund for the Doctoral Program of Higher Education
(20120041120022), the Fundamental Research Funds for the
Central Universities (DUT13LK29).
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