Copper nanoparticle/carbon quantum dots hybrid as green photocatalyst for high-efficiency oxidation of cyclohexane

Article abstract of DOI:10.1039/c5ra04753f

To develop green catalysts for cyclohexane oxidation with high efficiency and high selectivity is a trend in nanotechnology and nanocatalysis. In this work, we demonstrate that copper nanoparticles/carbon quantum dots (Cu/CQDs) hybrid as photocatalyst exh

Full text of DOI:10.1039/c5ra04753f

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DOI: 10.1039/C5RA04753F
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Copper nanoparticle/carbon quantum dots hybrid as green
photocatalyst for high efficiency oxidation of cyclohexane
Shi Qiao,‡a Baohu Fan,‡b Yanmei Yang,a Naiyun Liu,a Hui Huang*a and Yang Liu*a
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
5
DOI: 10.1039/b000000x
To develop green catalyst for cyclohexane oxidation with high efficiency and high selectivity is the
tendency in nanotechnology and nanocatalysis. In this work, we demonstrate the copper
nanoparticles/carbon quantum dots (Cu/CQDs) hybrids as photocatalyst exhibit excellent catalytic
activity in oxidation of cyclohexane (the conversion based on cyclohexane to 50.2 % and the selectivity to
10 cyclohexanone about 78.3 %) under a low temperature with tert-butyl hydroperoxide as oxidant. It is
worth mentioning the Cu/CQDs hybrid photocatalyst can be implemented in efficiently catalytic
oxidation of cyclohexane under a mild condition (60 ℃), which may provide a cogent pathway for the
development of high-performance catalyst for C-H oxidation.
multipurpose component in the design of novel photo-catalyst.25
So, the CQDs may be used as a powerful energy-transfer
component in the design of superior photocatalyst for the
oxidation of cyclohexane. With respect to the oxidant, tert-butyl
50 hydroperoxide (TBHP) is a preferable one, because of the
simplicity of handling and efficient performance in the oxidation
reaction. The use of TBHP could significant promote the catalytic
oxidation reaction.
Herein, we present the design and preparation of Cu/CQDs
55 hybrids as photocatalyst for oxidation of cyclohexane under
visible light. The Cu/CQDs hybrids exhibit efficient
photocatalytic activity with the conversion of cyclohexane up to
50.2 %, while the selectivity of cyclohexanone can reach to
78.3 %. Compared with traditional catalysis oxidation systems of
60 cyclohexane, the present Cu/CQDs nanoparticals are able to
efficiently catalyze the alkane oxidation with TBHP as an
efficient oxidant under a mild condition, demonstrating the great
potential of such an approach for C-H oxidation.
15 1. Introduction
The oxidation products of alkanes, such as, cyclohexanol and
cyclohexanone, are important chemical raw materials and
intermediates in chemical industry. But, the direct and efficient
oxidation of available alkanes (cyclohexane, etc.) remain an
20 ambition goal yet to be achieved.1-8 In general, the chemical
inertness of the hydrocarbons leads a huge challenge to the
activation of its C-H bond, usually requiring harsh conditions,
such as high temperature and pressure, which make the process
very low energy utilization.9 To overcome these difficulties,
25 many investigations have been done to develop more efficient
catalyst systems for the cyclohexane catalytic oxidation. A
zirconium complex bonded to modified carbamate silica gel gave
a
product distribution ratio of 6.6:1 of cyclohexanol-
cyclohexanone mixture with 21 % conversion at 200 °C.10
30 Chromium containing complex CrCoAPO-5(CH3COOH) gave
50 % conversion with some 55 %, 8 %, 15 % and 22 %
selectivity towards cyclohexanol, cyclohexanone, adipic acid and
others at 115 °C and 1 MPa under oxygen.11 So far, those reaction
processes require high temperature or pressure,12,13 also still
35 suffer from low yield, selectivity and high cost.14
2. Experimental Section
65 2.1 Chemical reagents
Unless otherwise noted, all materials were obtained from
commercial suppliers and used without further purification. All
chemicals were purchased from Sigma–Aldrich.
Copper nanoparticles (CuNPs) exhibit high catalytic activity
and much cheaper than precious metals
15-17
(like Au or Pt),
which can typically provide highly active centers, having a great
promising in catalysis.18-20 At the same time, the deposition of
40 metal nanocrystal on carbon materials for catalytic reactions are
meaningful works.21, 22 On the other hand, carbon quantun dots
(CQDs) also display great potential applications in nanocatalysis
due to their distinctive photo-physical and photoelectrochemical
properties,23, 24 especially they can act as photo-induced electron
45 acceptors or donors. More important, CQDs can be used as a
2.2 Preparation of CQDs
70 CQDs were synthesized through a method of electrochemical
ablation of graphite, which was reported by our group.26 In the
synthetic process, the graphite rod (99.99%, Alfa Aesar Co. Ltd.,
13 cm in length and 0.6 cm in diameter) was inserted into the
ultrapure water (18.4 MΩ cm-1, 600 mL), placed parallel to each
75 other graphite rod as the counter-electrode with a separation of
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7.5 cm. Static potential of 30 V was applied to the two electrodes
were added into the above three round flasks, respectively. The
oxidation products were analyzed by GC and GC-MS.
using a direct current power supply for 120 h with continuous
stirring. Then, a dark-yellow solution was obtained in the reactor.
The solution was filtered with slow-speed quantitative filter paper
for two times, and the resultant solution was centrifuged (22000
rpm) for 30 min. After that, the CQDs can be obtained.
View Article Online
DOI: 10.1039/C5RA04753F
60 2.8 Characterization
5
Transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) images of the samples were obtained with a FEI-
Tecnai F20 (200 kV) transmission electron microscope (FEI).
The Raman spectra were performed on an HR 800 Raman
2.3 Preparation of CuNPs
CuNPs were prepared by a chemical reduction method. A
solution (30 mL) containing copper (II) nitrate pentahydrate 65 spectroscope equipped with a synapse CCD detector. The
10 Cu(NO3)2·5H2O (0.01 M) and ascorbic acid (0.01 M) were
prepared in a conical flask under continuous vigorous stirring.
Then ice-cold freshly NaBH4 solution (1 mL, 0.1 mol L−1) was
added into the above solution. The resultant solution turned pink
spectrograph used 600 g·mm-1 gratings and a 633 nm He-Ne laser.
UV-visible spectra were taken on an Agilent 8453 UV-vis Diode
Array Spectrophotometer. The fluorescent spectrophotometry
study was recorded on a Horiba Jobin Yvon (FluoroMax 4)
immediately after adding NaBH4, indicating the formation of 70 Luminescence Spectrometer with slit of 3 nm, and the power of
15 CuNPs.
the excitation light is 150 W (ozone-free Xe lamp). The X-ray
powder diffraction (XRD) was characterized by using an
8/X‘Pert-ProMPD (Holand) D/max-γA X-ray diffractometer with
Cu Kα radiation (λ = 0.154178 nm). X-ray photoelectron
75 spectroscopy (XPS) spectra were recorded with a KRATOS Axis
2.4 Preparation of Cu/CQDs hybrids
The Cu/CQDs hybrids were prepared through dropwise addition
of Cu(NO3)2 solution (2 mL, 0.02 M) to CQDs solution (15 mL)
with vigorous magnetic stirring at room temperature. After
20 stirring at room temperature for about 1 h, a solution of NaBH4
(0.4 mL, 0.05 M) in deionized water was added to the mixture
under continuous strong stirring for about 60 min. The above
solution turned gray-blue immediately when added NaBH4
solution. The Cu/CQDs composites were separated and washed
25 with deionized water by centrifugation. After that, the Cu/CQDs
can be obtained.
ultra DLD X-ray photoelectron spectrometer with
a
monochromatised Al Ka X-ray (hν = 1486.7 eV). The GC were
studied in a Varian 3400 GC column with a cross-linked 5%
PhMe silicone column (25 m × 0.20 mm × 0.33 μm) and a FID
80 detector. While the fluorescence lifetimes of the CQDs were
measured by FluoroLog 3-211-TCSPC.
3. Results and Discussion
2.5 Preparation of CuO/CQDs hybrids
3.1 Characterizations of Cu/CQDs hybrids
The CuO/CQDs hybrids were prepared through dropwise
addition of Cu(NO3)2 solution (3 mL, 0.02 M) into a conical flask
30 containing 0.2 mL sodium dodecyl benzene sulfonate (SDBS)
solution (0.1 mM) with vigorous magnetic stirring at room
temperature. After 10 min, NH4HCO3 (4 mL, 0.04 M) solution
was added into the flask with a speed of 0.5 mL•min-1. After 1 h,
5 mL CQDs solution was added to the above flask with
35 continuous strong stirring and heating. After boiling the above
solution 6 h, the mixture was cooled to room temperature. The
above mixture was separated by centrifugation and washed with
deionized water. After that, the products were dried at 100 ℃ for
30 min. Then, the CuO/CQDs hybrids can be obtained.
40 2.6 Photocatalytic oxidation of cyclohexane using Cu/CQDs
hybrids
In the photocatalytic experiments, TBHP (10 mL) as oxidant was
added dropwise to the mixture of Cu/CQDs hybrids (20 mg) and
cyclohexane (10 mL) in a three-necked flask (50 mL) with water
45 condenser and continuous magnetic stirring under visible light
(xenon lamp with visible-CUT filter to cut off light with a
wavelength of λ < 420 nm) or dark. The oxidation products were
analyzed
by
gas
chromatography
(GC)
and
gas
chromatography−mass spectroscopy (GC-MS).
50 2.7 Reactive species trapping experiments
The TBHP (10 mL) as oxidant was added dropwise to the
mixture of Cu/CQDs composites (20 mg) and cyclohexane (10
mL) in a three-necked flask (50 mL) with visible light irradiation
and continuous magnetic stirring. Then three different radical
55 scavengers, tert-butanol (5 mL), benzquinamide (BZQ, 5 mL)
and disodium ethylenediaminetetraacetate (Na2-EDTA, 5 mL),
85 Fig. 1 (a and b) TEM and HRTEM images of Cu/CQDs, inset in Fig. 1a is the
particle sizes histogram of Cu/CQDs. (c-f) Raman spectra, FTIR spectra, UV-
vis absorption spectra and XRD patterns of CQDs (black line) and Cu/CQDs
(red line), respectively.
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Cu/CQDs hybrids were synthesized by
a
simple and
In order to analyze the elements composition of Cu/CQDs
convenient method at room temperature. Fig. 1a shows the 60 hybrids, X-ray photoelectron spectroscopy (XPS) was carried out.
View Article Online
transmission electron microscopy (TEM) image of the obtained
Cu/CQDs hybrids. The resultant Cu/CQDs hybrids appear
spherical dots, indicating the hybrids have a similar morphology
and dispersed well with the size in the range of 4-7 nm (inset in
Fig. S1a shows the XPS survey full sDpOecI:tr1u0m.103o9f/CC5RuA/C04Q7D53sF
hybrids, which exhibit the as-obtained sample contains carbon,
copper and oxygen elements without others impurities. The high-
resolution C 1s XPS spectrum of Cu/CQDs hybrids displays in
5
Fig. 1a). Fig. 1b displays the high-resolution transmission 65 Fig. S1b, in which the peaks at 288.7, 286.4 and 285.1 eV
electron microscopy (HRTEM) image of Cu/CQDs hybrid, which
exhibits the clear lattice spacing is around 0.212 nm for CQDs
respectively attribute to C=O, C-OH and C-H species.36-38 The
high-resolution O1s XPS spectrum of Cu/CQDs hybrids, shown
in Fig. S1c, can be mainly divided into three peaks located at
about 534.6, 533.9 and 531.8 eV, which may be ascribed to C-H,
10 corresponded to the crystallographic planes of graphitic carbon.27
While the lattice pitch of 0.208 nm corresponds to the (111)
crystallographic plane of Cu. The above results confirmed that 70 C=O and C-O, respectively.39
the formation of Cu/CQDs hybrids. Considering the good
3.2 Catalytic oxidation of cyclohexane using Cu/CQDs
catalytic activity of CuNPs and the excellent photo-physical and
15 photoelectrochemical of CQDs, we suspect that Cu/CQDs
hybrids may be a promising candidate of novel photocatalyst.
In order to further confirm the formation of Cu/CQDs hybrids,
a series of tests were carried out. The Raman spectra (λex = 633
nm) of Cu/CQDs and CQDs were measured shown in Fig. 1c. As
20 shown, the CQDs (black trace) exhibit the typical D-band and G-
band of carbon located at about 1341 and 1610 cm-1, respectively.
The D-band is disorder-induced because of the defects,
dislocations and lattice distortions in carbon structures.28, 29 The
G-band can be interpreted as the stretching vibration mode of
25 graphite crystals.30 The presence of Cu NPs in the Cu/CQDs
hybrids led to the enhanced Raman signal based on Surface-
Enhanced Raman Scattering (SERS).31, 32 So, the Raman signal of
Cu/CQDs is higher than that of CQDs. Moreover, it can be found
that the ratio of ID/IG for Cu/CQDs is higher than that of CQDs,
30 which indicated the presence of Cu NPs led to the increase of
disorder degree in Cu/CQDs hybrids compared with CQDs. The
FTIR spectrum of Cu/CQDs hybrids, shown in Fig. 1d (red line),
displays four prominent peaks. The peak at about 3400 cm-1
corresponds to the O-H stretching mode. The peaks at about 1700,
35 1440 and 1200 cm-1 indicate the presences of carbonyl (C-O)
groups, the C = C stretch of polycyclic aromatic hydrocarbons,
and the epoxide/ether C–O–C, respectively.26 These results show
that the Cu/CQDs hybrids have abundant carboxyl and hydroxyl
groups on their surfaces, which are benefit for the synthesis
40 nanocomposite with CQDs as a multipurpose component.
Fig. 1e shows the UV-vis absorbance spectra of CQDs (black
line) and Cu/CQDs hybrids (red line) in aqueous at room
temperature. As shown, the Cu/CQDs displays two strong
absorption peaks located about 230 nm and around 520 - 640 nm,
45 which represent the typical absorption of an aromatic π system
and the surface plasmon resonance of CuNPs, respectively.33, 34
The UV-vis absorption spectra results demonstrate the Cu/CQDs
composite can absorb significantly more visible light, which is
more useful in photocatalysis. These evidences indicated that
50 Cu/CQDs composites may have excellent catalytic activity for the
target reactions. The X-ray diffraction (XRD) patterns of
Cu/CQDs and CQDs are shown in Fig. 1f. The characteristic
diffraction peaks marked by a black circle (●) can be readily
indexed to (111), (200) and (220) planes of metallic Cu,35
55 respectively. The broad diffraction peak at 26° marked by a black
squares (■) is the characteristic peak of carbon. The above results
further confirm the Cu/CQDs hybrids have been successfully
obtained.
hybrids.
To investigate the photocatalytic activity of Cu/CQDs hybrids,
the cyclohexane oxidation with Cu/CQDs hybrids as
75 photocatalyst was performed under visible light and dark. Fig. 2
shows
the
relationships
between
conversion
of
cyclohexane/selectivity to cyclohexanone and reaction time used
Cu/CQDs hybrids as photocatalyst under visible light and dark.
The detailed conversion of cyclohexane and selectivity to
80 cyclohexanone are shown in Table S1 and S2. As shown in Fig. 2
and Table S1, after 48 h under visible light, the conversion of
cyclohexane and the selectivity to cyclohexanone reach 50.2 %
and 78.3 %, respectively. While under dark, the conversion of
cyclohexane and the selectivity to cyclohexanone achieved 4.9 %
85 and 73.3 %, respectively. These results indicate the Cu/CQDs
hybrids are efficient photocatalyst under visible light.
Fig. 2 Reaction time dependence of conversion of cyclohexane/selectivity to
cyclohexanone with Cu/CQDs as photocatalyst under visible light irradiation
90 (●) and dark (■).
Table 1. Photocatalytic oxidation of cyclohexane with different catalysts
under visible light.
Selectivity (%)
Conversion
(%)
TOF
(h-1)
Catalysts
No catalyst
CQDs
Cyclohexanone ∑selC6
--
--
--
--
--
8.1
72.7
70.2
78.3
73.2
94.1
92.4
95.7
96.3
CuNPs
6.5
36.4
246.1
56.3
Cu/CQDs
Cu&CQDs
50.2
10.1
To further confirm the photocatalytic activity of Cu/CQDs
hybrids, series of control experiments for cyclohexane
95 oxidation were carried out with different catalysts. Table 1
a
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exhibits the oxidation of cyclohexane with different catalysts (no
catalyst, CQDs, CuNPs, Cu/CQDs hybrids and the physical
mixture of CuNPs and CQDs (Cu&CQDs)) under visible light.
The blank reactions (without catalyst and nitrogen saturation
treatment) would not produce any oxidation products, which
confirmed the necessity of adding catalyst to oxidation
cyclohexane. When CQDs and CuNPs as photocatalysts, the
conversions of cyclohexane reach 8.1 % and 6.5 %, respcetively.
While the selectivity to cyclohexanone up to 72.7 % and 70.2 %,
View Article Online
DOI: 10.1039/C5RA04753F
5
50
10 respectively. In remarkable contrast, when Cu/CQDs hybrids
were used as catalysts, the conversion of cyclohexane (50.2%)
and the selectivity of cyclohexanone (78.3%) are obviously
higher than that of CQDs and CuNPs. To furhter comparision, the
physical mixture of CuNPs and CQDs (Cu&CQDs) was used as
15 photocatalyst. As shown in Table 1, the Cu&CQDs show higher
conversion and selectivity than that of CQDs and CuNPs, but
much lower than that of Cu/CQDs hybrids. The catalytic activity
of Cu/CQDs was significant higher than CQDs, CuNPs and
physical mixture Cu&CQDs. The turn-over-frequencies (TOF)
20 was further calculated by determining the moles of product
formed (or reactant consumed) per unit time per mole of available
active sites (based on Cu) on the catalyst.40 As shown in Table 1,
the TOF of Cu/CQDs is higher than that of other catalysts, which
indicates the catalytic activity of Cu/CQDs is higher than other
25 catalysts. The above results further indicate the Cu/CQDs hybrids
possess efficient photocatalytic activity. More important, the
photocatalytic activity of CQDs are higher than that of CuNPs,
which demonstrated that CQDs play a crucial role in catalytic
reaction.
Fig. 4 Relationship between the conversion of cyclohexane/selectivity of
cyclohexanone and cycle times with Cu/CQDs as photocatalyst over ten cycles.
To test the repeatability of Cu/CQDs hybrids as photocatalyst,
ten times cyclohexane oxidation were performed with the
55 recycled catalyst shown in Fig. 4. As shown, after recycling 10
times, the Cu/CQDs still keep high catalytic activity with nearly
constant conversion and selectivity. Moreover, the filtrate
solution test (an additional 36 h of reaction after removing the
catalytst from the reaction medium) showed no catalytic activity,
60 indicating that leaching did not play an important role in the
present system. The above results demonstrate Cu/CQDs are
indeed efficient photocatalyst in the present reaction system.
3.3 Proposed mechanism study
Based on the above experimental results, the efficient catalytic
65 performance of Cu/CQDs might be due to their structural features.
The surface plasma resonance of CuNPs can enhance the visible
light absorption. When the irradiation, the hydroxyl radical (HO•)
group is quickly generated from TBHP,41 which serves as a
strong oxidant for the conversion of cyclohexane to
70 cyclohexanone in the mild condition. Simultaneously, the CQDs
are excellent as both electron donors and acceptors, which can
further enhance the photocatalytic activity. In this photocatalytic
system, the CuNPs and CQDs as important components play the
synergistic catalysis for the oxidation of cyclohexane. To confirm
75 the HO• is the main active oxygen specie and responsible for the
selective oxidtative raction, three different scavengers, tert-
butanol (a HO• radical scavenger), benzquinamide (BZQ, a O2-•
radical scavenger) and disodium ethylenediaminetetraacetate
(Na2-EDTA, a hole scavenger) were used in the reactive species
80 trapping experments. Fig. 5 displays the photocatalytic results
when three different scavengers were added. The addition of tert-
butanol significantly reduced the conversion after 48 h reaction,
while the introductions of BZQ and Na2-EDTA, the conversion
nearly remain the same, which confirm that the HO• is the active
85 oxygen species in this photocatalytic oxidation process.
30
Fig. 3 (a) Wavelength dependence of the conversion with Cu/CQDs hybrids as
photocatalyst drived by different lighting conditions: blue (400 nm), green
(525 nm), red (620 nm), and IR (760 nm) light, the xenon lamp was as the light
source with CUT filters to get light of different wavelengths. (b) Light
35 intensity dependence of the conversion (black line)/selectivity (red line) of the
cyclohexane oxidation with Cu/CQDs as photocatalyst.
The Cu/CQDs photocatalyst driven by different wavelength
lights was further carried out with the cyclohexane oxidation (Fig.
3a). The highest conversion for oxidation of cyclohexane was
40 obtained when irradiated by red light (620 nm), whose
wavelength matches the surface plasma resonance zone of CuNPs
(540 - 640 nm).26 At the same time, the oxidation of cyclohexane
were carried out under different light source intensity (Fig. 3b). It
was found that the conversion and source intensity are in linear
45 relationship when irradiated with weak light (before 30 mw/cm2).
While with intense light (after 60 mw/cm2), the conversion is
almost a constant. The selectivity was almost remain the same.
These could because the catalyst active sites were fully activated
when irradiated by intense light.
Fig. 5 Photocatalysis activities of Cu/CQDs in reactive species trapping
experiments with three types of reactive species scavengers.
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To investigate the photoinduced electron transfer properties of
significantly decreased (conversion down from 32.3 % to 13.2 %).
CQDs, the photoluminescence (PL) properties of Cu/CQDs
hybrids were studied shown in Fig. S2. As shown in Fig. S2a and
S2b, CQDs can exhibit broad luminescence peak at about 550 nm
with the excitation at 485 nm, and the emission intensities were
quenched by the known electron donors (N, N-diethylaniline,
DEA, 0.03 M) and electron acceptors (2,4-dinitrotoluene, 0.03
M). It was found that the PL of CQDs excited by 485 nm with
emission located at 550 nm could be quenched by either electron
The above results indicated the Cu/CQDs hybrid as photocatalyst
View Article Online
55 exhibits efficient photocatalytic activity and stability.
DOI: 10.1039/C5RA04753F
For present catalysis system, compared with some other
catalytic reaction which demand harsh reaction conditions or used
precious metals,18 our experiments were conducted in
atmospheric pressure and a low temperature (60℃). Secondly, we
60 did not utilize any complicated equipments to carry out these
experiments. Thirdly, there are no organic solvents (like
acetonitrile, acetone, etc) added to improve the selectivity of
cyclohexanone. Therefore, the Cu/CQDs photocatalyst is a green
and efficient catalysis materials for the selective oxidation of
65 cyclohexane. Although conversion into cyclohexanone at this
stage of research is not very high, we believe that this is a proof
of concept that copper-based catalysts, could effective improve
the oxidation of C-H bond. How to further improve the
conversion and selectivety of selective oxidation cyclohexane
70 still needs further more exploration.
5
10 acceptor or electron donor molecules in solution, confirming the
CQDs are excellent as both electron donors and acceptors under
visible light. Fig. S2c and S2d show the luminescence decays of
CQDs with Stern-Volmer quenching constans (insets of Fig. S2c
0
and S2d, Ksv = τF kq) from linear regression of 22.7 and 37.2 M-1,
15 respectively. In the followed experiments, the interaction between
CuNPs and CQDs was further investigated. Fig. S3 exhibits the
PL spectra and luminescence decays of CQDs (black line) and
Cu/CQDs (red line). It can clearly see that the PL of CQDs can
obviously quench in the current system.
Fig. 6 demonstrate the high-resolution Cu 2p XPS spectra
before (Fig. 6a) and after (Fig. 6b) the oxidation of cyclohexane
with Cu/CQDs as photocatalyst. As shown in Fig. 6a, the Cu 2p
peak can be divided into two peaks located at 952.7 eV and 931.4
eV. After several times photocatalytic oxidation reaction, the
25 Cu/CQDs hybrids show the impurity (CuO). As shown in Fig. 6b,
two new peaks (962.6 eV and 943.1 eV) of CuO appeared. In
spite of this, the photocatalytic activity of Cu/CQDs hybrids
exhibit no obvious fall when the cycle experiments (Fig. 4). In
the following experiments, the CuO/CQDs hybrids were
30 synthesized to further invesitagte the photocatalytic activity for
cyclohexane oxidation. According to our experiments, the
conversion based on cyclohexane reach 23.9 % and selectivity to
cyclohexanone up to 70.6 %, which are lower than that of
20
4. Conclusions
We demonstrate the Cu/CQDs hybrids as photocatalyst exhibit
excellent catalytic activity in oxidation of cyclohexane (the
conversion based on cyclohexane to 50.2 % and the selectivity to
75 cyclohexanone about 78.3 %) under a low temperature (60 ℃)
with TBHP as oxidant. In comparison with previous catalytic
system for cyclohexane oxidation, there are three advantages to
the Cu/CQDs catalyst: (1) the synthesis pathway of Cu/CQDs
hybrids are fairly simple, requiring neither complicated devices
80 nor complex preparation processes; (2) the approach and raw
materials for manufacture Cu/CQDs hybrids are mild, cheap and
have potential for the large-scale production; and (3) the reaction
processes of cyclohexane oxidation requires neither high
temperature nor high pressure. The as-prepared Cu/CQDs hybrids
85 as the cheap and easily accessible photocatalyst provide a good
choice for green and high-performance oxidation of cyclohexane.
Cu/CQDs hybrids (50.2 % and 78.3 %). In fact, the CuO is also
43
35 an efficient catalyst in some reports.42,
The reasons for the
catalytic activity of CuO is lower than Cu are still in the research.
Acknowledgements
This work was supported by Collaborative Innovation Center of
Suzhou Nano Science and Technology, the National Basic
90 Research Program of China (973 Program) (2012CB825803,
2013CB932702), the National Natural Science Foundation of
China (51422207, 51132006, 21471106), the Specialized
Research Fund for the Doctoral Program of Higher Education
(20123201110018), a Suzhou Planning Project of Science and
95 Technology (ZXG2012028), the Natural Science Foundation of
Jiangsu Province of China (BK20140310), China Postdoctoral
Science Foundation (2014M560445), and a project funded by the
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD).
Fig. 6 High-resolution Cu 2p XPS spectra before (a) and after (b) the oxidation
of cyclohexane with Cu/CQDs as photocatalyst.
40
For further comparison between carbon dots and graphene as
substrates, we prepared the Cu/graphene hybrid as photocatalyst.
The catalytic activity of Cu/graphene hybrid for oxidation of
cyclohexane was performed under visible light (20 mg catalyst,
10 mL TBHP, and 10 mL cyclohexane with 48 h), which was
45 synthesized by adding NaBH4 (0.4 mL, 0.05 M) into a solution
containing Cu(NO3)2 (2 mL, 0.02 M) and graphene (1 mg) under
stirring 1 h. However, we found that the photocatalytic activity of
the Cu/graphene hybrid showed lower efficiency compared with
Cu/CQDs. The results are shown in Table S3. As shown, after 48
50 h, the conversion of cyclohexane and the selectivity to
cyclohexanone achieve 32.3 % and 69.5 %, respectively. After
recycling 3 times, the catalytic activity of the Cu/graphene
100 Notes and references
a
Jiangsu Key Laboratory for Carbon-Based Functional Materials &
Devices, Institute of Functional Nano & Soft Materials (FUNSOM),
Soochow University, 199 Ren'ai Road, Suzhou, 215123, Jiangsu, PR
China. E-mail: hhuang0618@suda.edu.cn, yangl@suda.edu.cn; Fax:
105 +86-512-65882846; Tel: +86-512-65880957
This journal is © The Royal Society of Chemistry year
Journal Name, year, vol, 00–00 | 5

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b
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