Au nanoparticles in carbon nanotubes with high photocatalytic activity for hydrocarbon selective oxidation

Article abstract of DOI:10.1039/c4dt01077a

High-efficiency and high-selectivity catalytic oxidation of alkanes under mild conditions with air is a major aim of current catalytic chemistry and chemical production. Despite extensive development efforts on new catalysts for cyclohexane oxidation, cur

Full text of DOI:10.1039/c4dt01077a

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Au nanoparticles in carbon nanotubes with high
photocatalytic activity for hydrocarbon selective
oxidation†
Cite this: Dalton Trans., 2014, 43,
12982
Juan Liu, Ruihua Liu, Haitao Li, Weiqian Kong, Hui Huang, Yang Liu* and
Zhenhui Kang*
High-efficiency and high-selectivity catalytic oxidation of alkanes under mild conditions with air is a major
aim of current catalytic chemistry and chemical production. Despite extensive development efforts on
new catalysts for cyclohexane oxidation, current commercial processes still suffer from low conversion,
poor selectivity, and excessive production of waste. Here, we present the design and synthesis of gold
nanoparticle/carbon nanotube (CNT) composites for high-efficiency and high-selectivity photocatalyst
systems for the green oxidation of cyclohexane. Remarkably, Au nanoparticles confined in carbon nano-
tubes (Au-in-CNTs) are photocatalytically active for the oxidation of cyclohexane with 14.64% conversion
of cyclohexane and a high selectivity of 86.88% of cyclohexanol using air and visible light at room temp-
erature. Given its diversity and versatility of structural and composition design, gold nanoparticle/CNT
composites may provide a powerful pathway for the development of high-performance catalysts and
production processes for green chemical industry.
Received 11th April 2014,
Accepted 4th July 2014
DOI: 10.1039/c4dt01077a
www.rsc.org/dalton
Noble metal nanoparticles (NPs) (i.e. Ag, Au) can strongly
absorb visible light due to their surface plasmon resonance
Introduction
Catalyzing partial oxidation of small hydrocarbons using mole- (SPR, it can be tuned by varying their size, shape and sur-
cular oxygen has been a very important process used to obtain rounding), working as an electron trap and active reaction site,
industrially important chemicals.1,2 The present commercial which gave rise to a new approach to efficient visible light
process for cyclohexane oxidation is carried out at around photocatalysts.10–15 In general, the visible-light plasmonic
423 K and 1–2 MPa pressure, affording ∼4% conversion and photocatalyst is a composite photocatalyst composed of noble-
∼60% selectivity to cyclohexanone and cyclohexanol over metal NPs and a polar semiconductor, but recently other
metal cobalt salt or metal-boric acid. Although many efforts materials such as carbon,16 graphene oxide, graphene, and
have been made to develop new catalysts (such as iron-based oxide insulators have also been used as supports and/or
catalyst systems,3–5 cobalt or manganese salts,6 copper species charge carriers to form plasmonic photocatalysts. On the other
within SBA-15,7 Fe2O3–TiO2,8 and Al-HMS9) to oxidize cyclo- hand, carbon nanotubes (CNTs) with high electron and
hexane under mild conditions, the present commercially- thermal conductivity, high surface area, and functionalizable
practiced process suffers from low yields (<8%), poor selectivity surfaces have evoked wide interest for catalytic
(<50%), as well as production of greenhouse gases and large applications.17–23 As a catalyst additive, CNTs promoted sub-
quantities of waste. So oxidizing cyclohexane with high conver- stantially the catalytic activities of catalysts. Particularly, their
sion and high selectivity under mild conditions is still a great channels are anticipated to provide an intriguing confinement
challenge for experimental study.
environment for metal catalysts and catalytic reactions, which
may provide opportunities for the development of new hetero-
geneous catalysts.24–28 Many researchers reported that metal
catalysts introduced into the CNT channels were essential to
achieve the optimized catalytic activity.29–33 This opens up an
opportunity to tune the catalytic performance via confinement
inside CNTs. Although metal catalysts confined in CNTs were
studied for many years, fewer reports focused on the light-
induced catalysis process based on this type of catalyst.
Considering the intriguing confinement environment of CNT
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for
Carbon-based Functional Materials and Devices, and Collaborative Innovation
Center of Suzhou Nano Science and Technology, Soochow University, Suzhou,
Jiangsu, China. E-mail: yangl@suda.edu.cn, zhkang@suda.edu.cn
†Electronic supplementary information (ESI) available: Details of experiment
description, HRTEM images of short CNTs, ESR spectra of the DMPO-•OH
(aqueous solution) adducts for the Au-in-CNT-H2O2 system, RRDE voltammo-
grams of Au-in-CNTs (Au-out-CNTs) and calculation of the H2O2 yield. See DOI:
10.1039/c4dt01077a
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Scheme 1 Au-in-CNTs composites as photocatalysts for selective oxi-
dation of cyclohexane using air as the oxidant under visible light.
channels and the SPR effect of metal NPs, the combination of
CNTs and metal NPs may be regarded as an ideal strategy to
construct the stable and efficient complex for high efficiency
oxidation of cyclohexane under visible light.
Fig. 1 (a) TEM image, high resolution TEM (HRTEM) pattern (inset) of
Au-in-CNTs; (b) Au nanoparticle size distribution is based on TEM
characterization shown in (a); (c) typical Raman spectra of CNTs (black
trace) and Au-in-CNTs (red trace); (d) the UV-Vis absorbance spectrum
of CNTs (black trace) and Au-in-CNTs (red trace).
Herein, we present the design and synthesis of Au NP/CNT
composites for high-efficiency and high-selectivity photo-
catalyst systems for the green oxidation of cyclohexane. In the
present catalyst system, Au nanoparticles were confined in
CNTs (Au-in-CNTs), which are photocatalytically active, and
can lead to the oxidation of cyclohexane with 14.64% conver-
sion of cyclohexane and a high selectivity of 86.88% of cyclo-
hexanol using air and visible light at 333 K (see Scheme 1). A
series of catalytic experiments suggested that the high conver-
sion and selectivity in the present photocatalyst system should
be attributed to the collective contribution of Au NPs and the
confinement reaction space in CNTs.
Raman scattering peaks, the former two strong peaks corres-
pond to the first-order D and G bands of CNTs. Here, the
G band splits into two peaks, in which the peaks centered at
1578.2 and 1606.8 cm−1 correspond to G− and G+ bands,
respectively. These two splitting peaks contribute to E2g mode
of graphite of metallic ≠ semiconducting type.36 The D-band
peak is attributed to the emergence of the crystallite size
effect, which makes some of the non Raman-active phonons
active.37 The two-order D peak (at 2644.7 cm−1) is also
obviously turned up when Au nanoparticles are confined in
CNTs. Fig. 1d shows UV-vis absorption spectra recorded of the
solid powder corresponding to the sample shown in Fig. 1a.
The bare CNTs exhibit an obvious sharper peak centered at
around 300 nm which derives from the CvC structure of
mixed acid treated CNTs.38 Compared with that of CNTs, the
UV-Visible absorption spectrum of Au-in-CNTs displays typical
absorbance in both UV (at 262 nm) and visible regions (at
538 nm). Typically, Au particles of about 5 nm show one major
plasmon absorption peak at 510–520 nm. However, here, the
Au plasma band red shifted to a longer wavelength with a
broad peak exhibited at about 538 nm, while the CNT absorp-
tion band blue shifted to a shorter wavelength. This pheno-
menon probably is attributed to the electron transferred from
Au to connected CNTs.39,40
Results and discussion
Characterization of Au-in-CNT composites
In our experiment, the raw CNTs were treated with concen-
trated sulfuric and nitric acids, and the obtained products
were short-CNTs with open ends (see Fig. S1 in the ESI†). Au-
in-CNT composites were fabricated by addition of the short-
CNTs to HAuCl4 solution with vigorous magnetic stirring in
the dark at room temperature (see the Experimental section
for details). The transmission electron microscopy (TEM)
image of Au-in-CNTs (Fig. 1a) shows that the Au nanoparticles
with a size of about 3.6 nm (for the size distribution see
Fig. 1b) are evenly distributed within the CNT channels.
Fig. 1a inset pattern presents the high resolution TEM
(HRTEM) image of Au-in-CNTs. The encapsulated Au nano-
particles and carbon nanotubes exhibit typical lattice fringes
with d-spacings of 0.23 and 0.34 nm, in well agreement with
Catalytic oxidation of cyclohexane using Au-in-CNT
composites
the (111) planes of crystalline Au and (002) crystallographic The catalytic activity of the synthesized Au-in-CNTs is tested by
planes of graphitic carbon respectively. Fig. 1c shows the the oxidation reaction of cyclohexane. In detail, the mixture of
typical Raman scattering peaks of CNTs at 1329.5, 1578.2 and Au-in-CNTs composites (50 mg), cyclohexane (10 mL) and
2644.7 cm−1 34
in which the Raman signal intensity is tert-butyl hydroperoxide (TBHP, 0.5 mL, is used as a radical
,
enhanced by Au nanoparticles owing to their surface enhance- initiator) are mixed together in a round bottom flask (250 mL)
ment Raman scattering effects.35 Among the above three with water condenser and continuous magnetic stirring at
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Fig. 2 Time dependence of the conversion and selectivity of the oxi-
dation of cyclohexane with different materials as photocatalysts under
Xe lamp irradiation in the dark.
Fig. 3 Photocatalytic activities of Au-in-CNTs in reactive species trap-
ping experiments with three kinds of reactive species scavengers.
333 K, air as the oxidant. The oxidation products were analysed
by gas chromatography (GC). Fig. 2 shows the time depen-
dence of the conversion and selectivity of cyclohexane oxi-
dation to cyclohexanol with Au-in-CNTs as the photocatalyst
driven by visible light. It reveals that the conversion efficiency
increases with increasing reaction time, while the selectivity to
cyclohexanone is nearly constant at 86.9%. After 48 h reaction,
a high conversion efficiency of 14.7% and a high selectivity of
over 86.88% were achieved concurrently.
In general, HO• is a strong oxidant, and can easily oxidize
cyclohexane to cyclohexanone and cyclohexanol. A spin-trap-
ping electron spin resonance (ESR) test was carried out to
check the active oxygen species produced by the Au-in-CNT-
water system under visible light. As shown in Fig. S2,† typical
characteristic quartet peaks of the DMPO-•OH (DMPO,
dimethyl pyridine N-oxide) adduct appeared gradually upon
Fig. 4 Steady state voltammograms for the ORR, in air-saturated 0.1 M
KOH, at Pt disk (modified with Au-in-CNT)-Pt ring electrodes (RRDE) at
rotation rates of 100, 400, 900, 1600 rpm (from inner to outer). The Pt
ring was polarized at +0.5 V vs. SCE. Potential scan rate of the disk elec-
trode: 20 mV s−1
.
light irradiation of the aqueous Au-in-CNT-water system, rotation speeds. The top curves of each plot represent the
which suggested that HO• is the active oxygen species in this corresponding Pt ring current (IR) of the relevant disk elec-
−
photocatalytic oxidation process. For further confirming this trode (due to the oxidation of HO2 generated at the disk elec-
point, reactive species trapping experiments are carried out to trode). The Pt ring is potentiostated at 0.5 V. Obviously, the Pt
investigate the reactive oxygen species in the photocatalytic ring current (Iring) is clearly observed, indicating the contri-
oxidation process.41 In this part, three different scavengers, bution of the 2-electron reduction mechanism in the ORR. The
•
tert-butanol (a OH radical scavenger), BZQ (a O2−• radical sca- H2O2 yield (%) is 40.38%, calculated using eqn (1) in ESI.†
venger) and disodium ethylenediaminetetraacetate (Na2-EDTA,
Subsequently, the electrocatalytic ability of Au-in-CNTs
a hole scavenger), are used. The photocatalytic performances modified GC electrodes toward H2O2 decomposition is ana-
in Fig. 3 show that the addition of tert-butanol substantially lysed in a conventional three-electrode electrochemical cell
reduced the photocatalytic activity for the oxidation of cyclo- with Au-in-CNTs modified GC electrodes as the working elec-
hexane from almost 15% to 2% in 60 h. The introduction of trode, platinum wire as the counter electrode, and Ag/AgCl
BZQ and Na2-EDTA does not affect the photocatalysis results. (saturated KCl) as the reference electrode. Fig. 5 shows the
•
It implies that a OH radical is the main reactive species, while cyclic voltammograms of Au-in-CNTs modified GC electrodes
O2−• radicals and holes do not contribute to the photocatalytic in 0.05 M (pH = 7.4) phosphate buffer in the presence of
performance. The addition of tert-butanol traps the •OH radicals different concentrations (0, 0.01 and 0.05 M) of H2O2 respecti-
leading to low conversion of cyclohexane. It confirms that in the vely. It is obviously shown that the reduction in current inten-
buffer solution containing the Au-in-CNT catalyst, a larger sity of H2O2 at −0.80 V on Au-in-CNTs modified GC electrodes
number of HO• will be produced with visible light irradiation.
becomes much larger when increasing the H2O2 concen-
To further confirm the HO• evolution, a series of electro- tration. This phenomenon indicates that Au-in-CNTs modified
chemical measurements (with the rotating ring disk electrodes GC electrodes possess an excellent catalytic ability toward the
(RRDE)) were carried out.42 Fig. 4 shows the steady-state hydro- H2O2 decomposition. Thus, in the present system, the gene-
dynamic voltammograms (RRDE) for the ORR at the Au-in- ration of HO• can be concluded in Scheme 2. First, O2 in air is
CNT modified Pt disk in air-saturated 0.1 M KOH at different partly catalytically reduced to H2O2 by Au-in-CNTs under light
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transferred electrons to their connected carbon nanotube
walls. To further prove this point, the influences of light radi-
ation over catalysts are detected using I–t curves. Both the cata-
lysts were indeed able to generate significant photocurrent
under irradiation (in Fig. S6†). In 0.1 M Na2SO4 solution, both
Au-in/out-CNTs reveal photocurrent generation, but the photo-
current intensity of Au-in-CNTs is almost three fold more than
that of Au-out-CNTs.
Surface plasmon resonance effect study
Cyclohexane oxidation was further carried out with the Au-in-
CNTs composite catalyst driven by light of different wavelengths
(Fig. 7). It shows that the best conversion efficiency is obtained
under green light, whose wavelength matches the surface
plasmon resonance zone of Au nanoparticles (500–600 nm).
Fig. 5 Cyclic voltammograms of Au-in-CNTs modified GC electrodes
in 0.05 M (pH = 7.4) phosphate buffer in the presence of different con-
centrations of H2O2 at a scan rate of 50 mV s−1
.
Confinement effect study
For studying the confinement effect of carbon nanotubes, the
Au nanoparticle loading on the outer wall of CNTs (Au-out-
CNTs) is also prepared. The CNTs used in this experiment are
untreated with the concentrated sulfuric and nitric acids. The
location and particle size of Au are characterized by TEM ana-
lysis. The TEM image of Au-out-CNTs (Fig. 8a) clearly shows
that the Au nanoparticles are dispersed on the outer wall
surface of the CNTs. The average particle size of Au in Au-out-
CNTs is about 4.5 nm, which are slightly larger than that of
Au-in-CNTs. Fig. 8a inset pattern presents the HRTEM image
of Au-out-CNTs with typical lattice fringes of (002) crystallo-
graphic planes of graphitic carbon and (111) planes of crystal-
line Au respectively. In Fig. 8c, the Raman signals of CNTs are
also enhanced by the loaded Au nanoparticles. Fig. 8d shows
UV-Vis absorption spectra obtained of the solid powder corres-
ponding to the sample shown in Fig. 8a. For Au-out-CNTs, the
absorption peak at about 300 nm corresponding to bare CNTs
slightly blue shifts to 287 nm. In the visible light range,
a broad peak exhibited at 500–700 nm, which is the result
of surface plasmon resonance of different sizes of Au
nanoparticles.
Scheme 2 Schematic illustration of the possible pathways of hydroxyl
radical generation.
−•
irradiation via a 2-electron ORR process. Then the HO2
decomposes to free HO•, which is more stable and more active
than the initial H2O2.
SECM measurements of Au-in-CNTs composites
Printed Au-in-CNTs spots are prepared as described in the
Experimental section. The ultramicroelectrode (UME) was first
positioned at about 15 μm from the substrate surface using
approach curves in K4Fe(CN)6 solution (4 mM). To begin with,
lateral scans are performed over Au-in-CNTs spots in the X-axis
direction as shown in Fig. 6. The mapping of these spots is
then imaged in two dimensions. Comparing these two SECM
curves (in Fig. 6), there is a significant increase in the
measured current with light radiation (red trace) as the UME
passing over the Au-in-CNTs spots, which indicate that as a
result of their photoinduced current enhancement on the
CNTs’ surface. The present current enhancement of CNTs by
light irradiation further confirmed that the internal Au NPs
Fig. 7 Excitation wavelength dependence of the conversion of the oxi-
dation of cyclohexane with Au-in-CNTs composites as photocatalysts
by different lighting conditions: blue (420 nm), blue-green (470 nm),
green (540 nm), and red (633 nm), using high power (>1 W) LED as the
light source.
Fig. 6 SECM mapping and X-scan lines of Au-in-CNTs spots with light
radiation (top part) and without light radiation (bottom part) scanned at
a speed of 1 μm s−1
.
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Table 1 Conversion of cyclohexane and the cyclohexanol selectivity
over different catalystsa
Selectivity [%]
Catalyst Au content Conversion
Entry
[mg]
[mg]
[%]
CyvO Cy-OH
Au NPs
Au-in-CNTs
0.05
50
0.05
0.05
0.05
0.05
0.05
0
0.94
9.29
12.79
5.94
13.56
31.25
0
80.39
86.88
70.28
62.99
47.01
0
14.64
3.45
2.65
3.16
0
0
0
Au-out-CNTs 50
Controlb
Controlc
Controld
Controle
Controlf
50
50
0
50
50
0.05
0
0
0
0
0
a Reaction conditions: 333 K, in air, 48 h, with a Xe lamp, 10 ml
cyclohexane, 50 mg catalyst, 0.5 ml tert-butyl hydroperoxide (TBHP).
b Au-in-CNTs as the catalyst, without the Xe lamp. c Au-out-CNTs as the
catalyst, without the Xe lamp. d Without any catalyst. e Without TBHP.
f Bare CNTs.
Fig. 8 (a) TEM image, high resolution TEM (HRTEM) pattern (inset) of
Au-out-CNTs; (b) Au nanoparticle size distribution is based on TEM
characterization shown in (a); (c) typical Raman spectra of CNTs (black
trace) and Au-out-CNTs (red trace); (d) the UV-Vis absorbance spectrum
of CNTs (black trace) and Au-in-CNTs (red trace).
particles confined in CNTs have a significant impact on the
catalytic behavior. The conversion of cyclohexane at 48 h was
elevated from 3.45 to 14.64%, corresponding to a fourfold
increase when Au nanoparticles were confined in CNTs. Mean-
while, the cyclohexanol (Cy-OH) selectivity of Au-in-CNTs
reaches 86.88%, indicating a promising selective production of
cyclohexanol. Both catalysts show higher conversion than the
pure Au catalyst (entry 1). In a word, CNTs as carriers play an
important role in the selective catalysis oxidation process of
cyclohexane. Light radiation is another factor for affecting the
conversion and selectivity of Cy-OH (entry 4, control experi-
ment b). Without Xe lamp emission, the conversion of cyclo-
hexane with Au-in-CNTs is much lower than that under
radiation, even lower than using Au-out-CNTs, which should
be due to Au nanoparticles inside lacking the plasmon effect,
and it is harder to contact with the reactant than outer Au
nanoparticles. Under the conditions where the catalyst is not
used, there is no Cy-OH or CyvO found in the product (entry
5, control experiment c). There is also no target product
obtained when TBHP is absent (entry 6, control experiment d).
In the following experiments, RRDE experiments were also
performed with the Au-out-CNTs modified Pt disk electrode.
As shown in Fig. 10, the Pt ring current (Iring) is also clearly
observed, indicating the contribution of the 2-electron
reduction mechanism in the ORR. The H2O2 yield (%) of
Au-out-CNTs catalysis ORR is 37.12%, which is smaller than
Au-in-CNTs. A comparative inspection of the RRDE voltammo-
grams in Fig. S3† reveals that the onset potential of the disk
current flow corresponding to the ORR at the Au-in-CNTs
modified electrode is positively shifted compared to the case
of the Au-out-CNTs modified Pt disk.
In the following catalytic experiments, a mixture of Au-out-
CNTs composites (50 mg), cyclohexane (10 mL) and TBHP
(0.5 mL) was treated in a round bottom flask (250 mL) using
air as the oxidant and a water condenser under continuous
magnetic stirring at 333 K. The oxidation products were ana-
lyzed by gas chromatography (GC). Fig. 9 shows the time
dependence of the conversion and selectivity of cyclohexane
oxidation to cyclohexanol with Au-out-CNTs as the photo-
catalyst driven by visible light. It reveals that the conversion
efficiency increases with increasing reaction time, while the
selectivity to cyclohexanone is about 70.28%. After 48 h reac-
tion, a conversion efficiency of 3.45% is achieved which is
much lower than that of Au-in-CNTs.
Proposed mechanism study
For further studying the proposed mechanism, a series of
control experiments are carried out. Table 1 lists the catalytic
performances of different catalysts. The contrast of Au-in-CNTs
(entry 2) and Au-out-CNTs (entry 3) indicates that Au nano-
Based on the above results and discussions, the proposed
mechanism for the photoinduced catalytic ability of the
present catalyst is described as follows. The surface plasmon
resonance of Au NPs enhances the light absorption of Au-in-
CNT composites. When Au NPs were assembled on CNTs
(inside or outside), the excitation of the SPR of the Au NPs
induced the charge transfer from the Au NPs to CNTs, which
Fig. 9 Time dependence of the conversion (black line) and selectivity
(red line) of the oxidation of cyclohexane with Au-out-CNTs as photo-
catalysts under Xe lamp irradiation.
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323 K in air. The average conversion rate of 14.525 and the
selectivity of Cy-OH of 87.660 are very close to the original
result (entry 7, control experiment f).
Conclusions
In summary, Au nanoparticles confined in carbon nanotubes
(Au-in-CNTs) are prepared successfully. Structural characteriz-
ation by various techniques reveals that the Au nanoparticles
are trapped into the short CNTs. The prepared catalyst, Au-in-
CNTs, is active for the oxidation of cyclohexane with 14.64%
conversion of cyclohexane and a high selectivity of 86.88% of
cyclohexanol under quite mild conditions (333 K, in air, 48 h,
with a Xe lamp, TBHP using as the initiating agent). The cata-
lyst is environmentally friendly. Au-in-CNTs decrease the
amount of noble metals and enhance the catalyst effect. More-
over, the catalyst is stable in the reaction system with excellent
repeatability. This synergetic confinement effect of metal
nanoparticles inside CNTs can lead to new advances and appli-
cations pertaining to functional materials in electronic, mag-
netic and catalytic fields, and related processes.
Fig. 10 Steady state voltammograms for the ORR, in air-saturated 0.1
M KOH, at Pt disk (modified with Au-in-CNTs)-Pt ring electrodes (RRDE)
at rotation rates of 100, 400, 900, 1600 rpm (from inner to outer). The
Pt ring was polarized at +0.5 V vs. SCE. Potential scan rate of the disk
electrode: 20 mV s−1
.
enhanced the separation of photo-induced holes and elec-
trons. At the interface of CNTs and Au NPs, O2 was reduced to
H2O2 and then decomposed into hydroxyl radicals (HO•) in the
presence of an initiator (TBHP). HO• serves as a strong oxidant
for conversion of cyclohexane to cyclohexanone. In the
process, the interaction between CNTs and Au NPs under
visible light plays a key role in the eventual high conversion
and selectivity. Confined Au nanoparticles proved surface
plasmon resonance with Xe lamp irradiation, by which the
active Au nanoparticles donate electrons to the attached
internal CNT wall. For the space limitation of CNTs, the Au NP
size in CNTs is smaller than Au NP loading on the outside
wall. It is one reason for the higher catalytic activity of Au-in-
CNTs. But the particle size is not the only factor that leads to
the significant difference in the catalytic performance of the
Au-in-CNTs and Au-out-CNTs catalysts. Besides that, confine-
ment of reaction intermediates within such nanochannels
might prolong their contact time with the catalyst as reported
in the literature.43
Acknowledgements
This work was supported by the National Basic Research
Program of China (973 Program) (2012CB825803,
2013CB932702), the National Natural Science Foundation of
China (51132006), the Specialized Research Fund for the Doc-
toral Program of Higher Education (20123201110018),
a
Suzhou Planning Project of Science and Technology
(ZXG2012028), and a project funded by the Priority Academic
Program Development of Jiangsu Higher Education
Institutions.
To evaluate the stability of the catalyst, the recycling of
Au-in-CNTs is performed for the catalyzed oxidation of cyclo-
hexane (as shown in Fig. 11). The catalyst is recovered by
centrifugation, washed with ethanol and water, and dried at
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