Shape- and size-controlled synthesis of Cu nanoparticles wrapped on RGO nanosheet catalyst and their outstanding stability and catalytic performance in the hydrogenation reaction of dimethyl oxalate

Article abstract of DOI:10.1039/c8ta07371f

Considerable interest has been paid in the last few years to the important environmentally benign catalytic hydrogenation reaction of dimethyl oxalate (DMO) to produce methyl glycolate (MG), ethylene glycol (EG) and ethanol (ETOH). SiO₂-supported noble metal elements, such as Au, Ag and Ru, have been nominated as efficient catalysts for the production of high MG selectivity. In this work, noble-metal-free Cu nanoparticles wrapped on reduced graphene oxide (RGO) nanosheets were synthesized using a green in situ sonochemical method in an aqueous medium and then applied without the calcination step as a robust catalyst in the DMO hydrogenation reaction. Moreover, the effects of ultrasound (US), ammonia (NH₄OH), a combination of both US and NH₄OH, and Cu loading (10, 25 and 45 wt%) on the morphology and catalytic performance of the produced Cu/RGO catalysts were studied in detail. Interestingly, transmission electron microscopy images revealed that a dramatic morphology evolution of the Cu catalysts from ultra small dots to elongated and leaf-like nanoparticles shapes occurred, depending on simple modification of the reaction parameters. Notably, the obtained catalytic results demonstrated that among the different produced six catalysts, the prepared Cu/RGO catalyst using 20 kHz of ultrasound and in the presence of ammonia with 25 wt% Cu loading displayed the highest MG selectivity (98.8%) at a relatively low reaction temperature (210 °C), while, the synthesized Cu/RGO catalyst with 45 wt% Cu loading exhibited the highest ETOH selectivity (94%) at 240 °C. Moreover, both catalysts showed long-term stability for at least 300 h while maintaining the high DMO conversion and selectivity ratio. The comprehensive characterization to the as-prepared, reduced and spent catalysts concluded that the synergistic ratio of Cu⁺/(Cu⁺ + Cu⁰), the high dispersion of Cu nanoparticles, the defective edges, sites and electron transfer provided from RGO were the main reasons for the obtained superior catalytic activity and long-term stability.

Full text of DOI:10.1039/c8ta07371f

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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DOI: 10.1039/C8TA07371F
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ARTICLE
Shape and size controlled synthesis of Cu nanoparticles-
wrapped on RGO nanosheet catalyst and their outstanding
stability and catalytic performance in the hydrogenation
reaction of dimethyl oxalate
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Mohamed Abbas a,b,*, Zheng Chen a,c and Jiangang Chen a,*
www.rsc.org/
Considerable interest has been paid in the last few years for the important environmental-benign catalytic hydrogenation
2
reaction of dimethyl oxalate (DMO) to produce methyl glycolate (MG), ethylene glycol (EG) and ethanol (ETOH). SiO -
supported noble metal elements such as Au, Ag and Ru have been nominated as efficient catalysts for the production of
high MG selectivity. In this work, noble-metal free, Cu nanoparticles-wrapped on reduced graphene oxide (RGO)
nanosheets were synthesized using green in-situ sonochemical method in aqueous medium and then applied without the
calcination step as robust catalyst in DMO hydrogenation reaction. Moreover, the effect of ultrasound (US), ammonia
4 4
(NH OH), combine both of US and NH OH, and Cu loading (10, 25 and 45 wt%) on the morphology and catalytic
performance of the produced Cu/RGO catalysts were studied in details. Interstingly, transmission electron microscopy
images revealed that a dramatically morphology evolution of cu catalysts from ultra small dots to elongated and leaf-like
nanoparticles shapes were produced, depending on the simple modifications of the reaction parameters. Notably, the
obtained catalytic results demonstrated that among the different produced six catalysts, the prepared Cu/RGO catalyst using
2
0 KHz of ultrasound and in presence of ammonia within 25 wt% of Cu loading displayed the highest MG selectivity
(98.8%) at relatively low reaction temperature (210ºC), while, the synthesized Cu/RGO catalyst within 45 wt% of Cu
loading exhibited the highest ETOH selectivity (94%) at 240 ºC. Moreover, both catalysts showed long term stability for at
least 300 h within maintained high DMO conversion and selectivity ratio. The comprehensive characterization to the as-
+
+
0
prepared, reduced and spent catalysts concluded that the synergistic ratio of Cu /(Cu +Cu ), the highly dispersion of Cu
nanoparticles, the defective edges, sites and electron transfer provided from RGO are the main reasons for the obtained
superior
catalytic
activity
and
long
term
stability.
material for the production of EG and ETOH through the
hydrogenation reaction of DMO with high selectivity and
1
. Introduction
6
-8
conversion ratio. However, the high thermodynamic constant
of the DMO hydrogenation reaction caused a difficulty to
produce high MG selectivity using comparable Cu-based
catalyst due to the subsequent hydrogenation of MG to EG and
Dwindling of the fossil fuels and the increased high
concern about the environmental pollution in worldwide has
pushes considerable interest from the academic and industrial
researchers toward the hydrogenation reaction of dimethyl
9
ETOH.
2
oxalate (DMO) gathered from syngas (CO+H ) as an
Therefore, noble metals such as Au, Ag and Ru
economical and environmentally-benign route for the
production of methyl glycolate (MG), ethylene glycol (EG) and
ethanol (ETOH). Notably, MG and EG have been immensely
used in various applications as an essential intermediate and
raw materials in the synthesis of polyester, lubricant, antifreeze,
polyglycolic acid, perfumes and pharmaceutical products,1-3
while, ETOH is an interesting feedstock as alternative synthetic
fuels for using in automobiles and also for the production of
valuable materials such as polymer and other chemicals.4,5 Cu-
based catalysts have been nominated as highly efficient
nanodots as active species were utilized to stabilize and
promote the Cu-based catalysts to achieve high MG selectivity
10-13
from the hydrogenation reaction of DMO.
However, they
are distant from the large scale and the real industrial
application due to the high cost of the noble metals. Besides,
using of various inorganic materials such as SiO , ZrO , Al O ,
2 2 2 3
HA and CNTs as support to Cu-catalyst and in absence of noble
metal as promoters was found to be a useless approach to
achieve high MG selectivity. Thus, developing of an alternative
support and catalyst materials for the production of high MG
selectivity without the needs for using any noble metal via the
DMO hydrogenation reaction is in utmost need and has drew
tremendous interests from the academic and industrial societies.
a.
b.
c.
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan, 030001, China.
Ceramics Department, National Research Center, 12622 El Behouth Str., Cairo,
Egypt.
University of Chinese Academy of Sciences, Beijing, China
E-mail: mohamed_abbas83@yahoo.com, mohamed@dgist.ac.kr,
chenjg@sxicc.ac.cn
Reduced graphene oxide nanosheets (RGO NSs), one of
the quite attracting 2D nanomaterials, have been confirmed its
extraordinary properties as robust support material in several
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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catalysis applications owing to its unique physical, chemical
and mechanical properties such as high carrier mobility, surface using modified Hummers method. Subs
area, excellent electrical and thermal conductivity, ease surface rapid ultrasonocation reaction of aqueous solution of GO and
modification through the abundant oxygen containing groups, Cu(NO .6H O is carried out in aqueous medium with 20 kHz
GO NSs is firstly prepared from graphite nanoflakes
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equently, in-situ and
DOI: 10.1039/C8TA07371F
1
9
3
)
2
2
1
4
edges defects, and adsorption capacity. For instance, RGO frequency and pulsed mode of 3 Sec (ON): 1 Sec (OFF) for 40
NSs are identified as outstanding carrier and support material in min. The pH of the reaction was adjusted using ammonium
different catalysis applications, including lithium ion batteries, hydroxide solution. After completion of the reaction, the
Fischer-Tropsch synthesis and photocatalytic conversion of ultrasonication was halted, and the precipitated CuO/RGO
into methanol.1
5-18
Therefore, from the viewpoint of real catalyst was collected using centrifugation and washed using a
CO
2
application, RGO-supported Cu nanoparticles is expected to mixture of ethanol and water, and then dried for 8h in normal
produce an outstanding performance in DMO hydrogenation drier at 90 °C. The prepared and non-calcinated CuO/RGO NPs
reaction. To the best of the author’s knowledge, Cu/RGO as catalyst could be used in DMO hydrogenation reaction after
catalyst in DMO hydrogenation reaction is not reported yet till reduced by H
2
at 280 °C into the active Cu/RGO catalyst.
now. Moreover, there are rarely studies reported so far for the Six Cu/RGO catalysts were synthesized in this study with
production of high MG selectivity using sole copper catalyst changing the Cu loading and reaction condition and given the
materials and without incorporation of noble metal as active following names: Cu/RGO (10 wt%), Cu/RGO (25 wt%)
species in the DMO hydrogenation reaction.
Cu/RGO (45 wt%), when the reaction performed in presence of
both Ultrasound and NH OH. While, the other 3 catalysts
4
In this work, morphology-controlled Cu nanoparticles- synthesized within 25 wt% of Cu loading and in presence and
wrapped on RGO NSs were successfully synthesized using 20 absence or combine of both ultrasound and ammonia and
kHz of high intensity ultrasound. Interestingly, Cu(NO
3
)
2
.6H
2
O
4
labelled as following: Cu/RGO(US), Cu/RGO(NH OH) and
and graphene oxide were appointed as precursor and aqueous Cu/RGO (US/NH
medium used as green and sole solvent, meanwhile, the
4
OH).
.
.
produced reactive free radicals such as H and OH during the 2.3. Analysis methods
reaction were obviates us from using any kinds of reducing
agent or surfactant in our environmentally-benign
The phases purity and crystal structure of the catalysts
sonochemical synthesis. A proposed reaction mechanism for were characterized by X-ray diffraction (XRD) patterns through
the sonochemical synthesis of Cu/RGO NSs catalyst was a Shimadzu XRD-6000 diffractometer and using Cu Kɑ
discussed in depth. The as-synthesized Cu/RGO nanoparticle as radiation (λ=1.5418Å) at 40 kV and 150 mA. The shape, size
novel catalyst without the pre-calcination step catalysts were and crystal planes of the catalysts were obtained using Hitachi
subjected to industrially relevant vapor-phase hydrogenation H-7650 (HRTEM, JEM 2100F), meanwhile, the chemical and
reaction of DMO at different reaction temperature. Moreover, elemental compositions of the catalysts were identified through
the effect of different parameters such as Cu loading (10, 25 the EDS mapping analysis (coupled with the TEM equipment).
and 45 wt%), the presence and absence of ultrasound and The physical properties of specific surface area(SBET) and pore
ammonia on the catalytic performance of Cu/RGO catalysts structure were tested through nitrogen adsorption-desorption
were investigated in details. Besides, the stability of the method (Tristar II3020), and the multipoint BET surface was
catalysts was studied for a long period of 300 h of reaction calculated in the range of 0.05 to 0.3 and with a relative
time. The as-prepared, reduced and spent catalysts were fully pressure based on the model of the Brunauer-Emmet-Teller.
characterized through TEM, HR-TEM, XRD, N
adsorption, XPS, Cu-LMM and H
-TPR to understand clearly according to the desorption branches of the isotherm and based
the relation between the crystal structure and the catalytic on the method of Barrett-Joyner-Halenda. Cu-LMM Auger
2
physical While, the curves for pore size distribution were drawn
2
performance.
analysis and X-ray photoelectron spectroscopy were used to
analyze the composition and the elemental valance of the
synthesized catalysts surface. H -TPR profiles for the as-
2
synthesized catalysts were obtained using Micromeritics
Autochem II 2920 instrument. The catalysts were firstly
reduced using gas mixture of (5% H to 90 % N ) within a flow
2
. Experimental
2
.1. Materials and device
2
2
-
1
o
rate of 50 mL min , and reaction temperature up to 800 C with
o
-1
3 2 2
Copper nitrate (Cu(NO ) .6H O), ammonia, sulphuric ramping at 10 C min .
acid (H
2
SO
4
), hydrochloric acid (HCl), sodium nitrate
) were purchased
(
NaNO ), potassium permanganate (KMnO
3
4
2
.4. DMO hydrogenation reaction test
from Sinopharm Chemical Reagent Co., Ltd, China. The
graphite flakes (around 325 mesh, 99.995%) was purchased
from Alfa Aesar company, USA. The employed ultrasonic
device is an ultrasonic processor (Ningbo Licheng Instrument
The catalytic performance test was performed using a
stainless steel tubular fixed-bed reactor. Typically, 0.3 g of the
as-peppered catalyst was sandwiched between two layers of
sand quartz and packed into a bed-reactor and firstly pre-
2
Co. Ltd, LC-1800) and a titanium horn with a 5 cm irradiating
surface area was equipped to the sonoreactor .The maximum
power of ultrasound device is 1800W and the piezoelectric
transducer supported by a 20 kHz generator and the reaction
temperature was performed at 300 K.
2 2
reduced in a stream of diluted hydrogen (10% H in N ) at 280
o
C under atmospheric pressure for 4 h with a flow rate of 30
-1
mL min , and then the catalyst bed was cooling to the room
temperature naturally. After that, 12.5 wt% DMO (purity >
.
9
H
9.5%) in methanol was pumped into the catalyst bed and pure
was fed into the reactor at a H /DMO molar ratio of 200 and
a system pressure of 2.5 MPa. The hydrogenation reaction test
2
2
2
.2. Sonochemical synthesis of Cu/RGO catalyst
-
1
2
was performed at H /DMO of 200 mol mol , WLHSVDMO of
1
-1
0
.257 g gcatal- h , meanwhile, various reaction temperatures
in the range of 200-240°C were used in this study. The
2
| J. Name., 2012, 00, 1-3
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collected spent catalysts were analyzed online through CuO/RGO to be mapping analysis, while Fig. 2VbiewsAhroticwlesOntlhinee
GC7890-MS5875C and using a gas fitted with a 30 m HP-5 EDS point analysis and 2C shows the ma le
capillary column and a flame ionization detector (FID). (C, O and Cu) in the catalyst jointly. Fig. 2d-f shows the
DOI: 10.1039/C8TA07371F
pping of all e ments
individual elemental maps. Remarkably, when we increased the
Cu loading to 45wt%, a dramatically evolution in the
morphology is obtained to be leaf-like shape of CuO NPs
wrapped on the RGO NSs as clearly shown in Fig. 3. Fig. 3c, d
shows the HRTEM images to the particles, and from the figure,
the calculated interplanar distances between the crystal lattice
fringes is 0.245 nm, which matched well with the crystal planes
of CuO (002).TEM-EDS mapping is performed to confirm the
homogenous and well-distributed of all elements in the
CuO/RGO catalyst as shown in Fig.4. However, when only US
3
. Results
3
.1 Proposed synthesis reaction mechanism
In this study, an in-situ and green sonochemical
approach is successfully adopted for the synthesis of
morphology controlled Cu nanoparticles-wrapped on RGO
nanosheets. The reaction is simply carried through the
application of 20 kHz of high intensity ultrasound into a
mixture of GO and Cu(NO
first, highly reactive free radicals are created from the reaction
between the ultrasound and water (H O+ultrasound → H•+ OH•),
and these radicals could integrate to produce strong reducing and
oxidizing agents (H•+H•→H
enhances the decomposition process of Cu(NO
into Cu atom clusters.
situ oxidation process to that Cu atom will produce the CuO
4
is applied to the aqueous medium and without NH OH, a
polydisperse particles size distribution within irregular shape or
amorphous-like is resulted, and the TEM images are provided
in the supplementary materials (Fig. S1), which reflecting the
importance of ammonia as pH controller during the synthesis
reaction. Whereas, when the reaction process is carried out
without ultrasound and using normal magnetic stirring, with the
addition of ammonia, elongated-like shape of CuO/RGO
catalyst is obtained within 50 nm of average particles sizes as
demonstrated in Fig. 5. Besides, thick layers of RGO sheets are
formed, which also reflected the importance of ultrasound to
prevent the re-aggregation and obtain thin layers.
3 2 2
) .6H O in aqueous medium. At
2
2
and OH•+OH•→H ), which
2
O
2
3
)
2
.6H O precursor
2
2
0,21
Subsequently we hypothesize that an in
2
2,23
nanoparticles.
4
However, in presence of NH OH along with the
aqueous medium, the reaction is proposed to be enhanced more and
a stable complex of [Cu(NH
2
+
3
)
4
]
is form. After that, the
functional groups situated on the edges and surfaces of the
aqueous solution of GO such as ketone, hydroxyl, epoxy and
carboxyl groups24 are proposed to work as anchor sites to CuO
NDs via the electrostatic binding, physisorption processes or
charge-transfer interaction, consequently, leads to in-situ
growth of CuO on the surface of RGO to form CuO/RGO NSs.
Noteworthy, with the respect to the most of the reported
conventional chemical reduction approaches for the synthesis
of metal/RGO nanocomposite, the created abrupt liquid jet
streams (~400 km/h) and shock waves produced from the
collapse of bubble during the ultrasonication reaction are
proposed to promote the direct attachment of the newly-formed
CuO NPs to the GO surface without any chemical linker or
surface functionalization to prevent the re-aggregation and
produce highly dispersed and uniform homogenous of
CuO/RGO catalyst.2
5,26
Moreover, as stated previously by Zhu
et al., the adsorbed bubbling gases on the surface of nuclei will
cause a reduction in the interfacial free energy of the solution
and the formed nuclei, thereby, barring the particles grain
growth, and thus a rapid particles nucleation is occurred within
a progressive in the grain growth, which leads to a boosting in
2
5
the synthesis process to smaller sizes.
Fig. 1 TEM images of (a, b) RGO NSs, (c, d) 10 wt% CuO/RGO
catalyst and (e, f) 25 wt% CuO/RGO Catalysts synthesized in
presence of ultrasound and ammonia.
3
.2. Characterization of the catalysts
Interestingly, when different reaction parameters were
used during the synthesis process such as the effect of
ultrasound, NH OH, and Cu loading, various morphologies of
4
Cu/RGO catalysts were obtained. Transmission electron
microscopy was used to monitor the morphologies evolution
according to the reaction parameters modifications. When both
4
of US and NH OH used in the reaction, a quite small size of
CuO NDs with an average particles size of 2 and 3nm are
obtained in both catalysts within Cu loading of 10 and 25 wt%,
respectively, as shown in Fig. 1. The HRTEM image in Fig. 1d
shows clearly the lattice fringes, which reflected the highly
crystalline structure of CuO NDs. The spatial distribution as
well as the outstanding homogeneous dispersion of CuO NDs
on RGO NSs is confirmed through the SEM-EDS mapping
analysis as shown in Fig. 2.Fig.2a revealed the SEM image for
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DOI: 10.1039/C8TA07371F
Fig. 2 SEM-EDS mapping analysis images of CuO/RGO catalyst
which display the elemental maps of C, O and Cu elements.
Fig. 4 EDS mapping analysis images of 45 wt% CuO/RGO catalyst
which display the elemental maps of C, O and Cu elements.
Fig. 3 TEM images of 45 wt% CuO/RGO catalysts synthesized in
presence of ultrasound and ammonia.
Fig. 5 TEM images of 25 wt% CuO/RGO catalysts synthesized in
presence of ammonia and without ultrasound.
The crystal structures and phases of GO, as-prepared and
reduced Cu/RGO catalysts synthesized in presence of US or
NH OH or both US/NH OH were characterized using XRD as
4 4
shown in Fig. 6. From the diffraction peaks of as-prepared
CuO/RGO catalysts in Fig. 6a, the first observation is the
completely vanished of any peaks related to GO in both
4 4
catalysts of CuO/RGO (NH OH) and CuO/RGO (US/NH OH),
which emphasizing the entirely reduction of GO to RGO and
the self-restacking of the as-reduced graphene sheets is
2
7,28
perfectly prevented.
However, the second observation is the
remain of some peaks of GO in the catalyst synthesized in
presence of ultrasound and absence of ammonia
(
4
Cu/RGO(NH OH)), which confirming that the sole ultrasound
is not enough to completely reduce the GO into RGO. While,
4
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the reduced Cu/RGO in the three catalysts exhibited diffraction
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peaks with high intensity at 2θ= 43.3°, 50.5° and 74.1° for the
DOI: 10.1039/C8TA07371F
2
metallic Cu (Card nu 04-0836) and at 2θ= 37° for Cu O (Card
0
+
nu: 35-1091) (Fig. 6b), suggesting a species of Cu and Cu are
co-existed in the catalyst after reduction with hydrogen and
maybe will have a s synergistic effect on the DMO
hydrogenation reaction as will be discussed below.
Nonetheless, the peaks intensity is identified very low in the
catalyst synthesized in the presence of only ultrasound.
Fig. 8 High resolution XPS spectra for reduced Cu/RGO catalysts
synthesized (a) in different reaction condition and (b) synthesized in
Fig. 6 XRD pattern for (a, b) As-prepared and reduced Cu/RGO presence of ultrasound and ammonia within different Cu wt% loading.
catalysts synthesized in different reaction conditions and (c,d) As-
prepared and reduced Cu/RGO catalysts synthesized in presence of
ultrasound and ammonia within different Cu wt% loading.
2
From the N adsorption/desorption isotherms of the Cu/
RGO catalysts as shown in Fig. 7, the isotherms are mostly
exhibited type IV, which is characteristic of the typical shape of
a mesoporous material. Besides, a rising in the isotherm of the
Fig. 6 c, d shows the XRD diffraction peaks of as-
prepared and reduced Cu/RGO catalysts synthesized in
presence of both US and NH OH and within different Cu
4
4
Cu/RGO(US/NH OH) at high relative pressure (>0.9) is
observed, which confirming the presence of macro-porous
loading (10, 25 and 45 wt%). Fig. 6c shows the XRD
diffraction peaks for the as-prepared CuO/RGO catalysts, and
from the figure, we can see clearly that the peaks intensity is
increased to reach the highest within Cu loading as expected
and the detected peaks at 2θ= 32.2°, 35.2°, 38.6° and 48.6° are
assigned to CuO phase in the three catalysts. However, the
reduced catalysts revealed similar diffraction peaks but within
different intensity at 2θ= 43.3°, 50.5° and 74.1° for the metallic
29
structure in the catalyst. The BET surface area and the pore
diameter of the six prepared catalysts are calculated and shown
in Table. S1 and Fig. 7. From the figure, it can be stated that the
specific surface area values and the pore diameter of Cu/RGO
catalysts are varied based on the synthesis method as well as
the different copper loading. The catalyst synthesized in
4
presence of both US and NH OH displayed the largest pore
diameter value, while, with the increment in the copper loading
from 10 wt% to 45 wt%, the BET surface area is decreased
2
Cu (Card nu 04-0836) and at 2θ= 37° for Cu O (Card nu: 35-
1
091) (Fig. 6d).
2
2
from 49.6 m /g to 37 m /g; and this kind of decreasing is maybe
understood in terms of increasing the covering area from Cu
nanoparticles in the surface of RGO NSs and consequently
decreasing the total surface area and also may be because the
dramatically increased of the particles size as shown in the
TEM images. Furthermore, the catalyst synthesized without
ultrasound (Cu/RGO(NH4OH) exhibited the lowest pore
diameter as well as surface area values, which reflecting the
importance of US.
On the other hand, to precisely identify the surface
components and valence state of the catalysts, XPS analysis
was performed to the as-synthesized and reduced Cu/RGO
catalysts. From the XPS survey spectra of the catalysts, the
major surface species in the Cu/RGO catalysts are composed
from Cu, O and C (Fig. S2 a, b). Fig. S2 c, d shows the high
resolution XPS spectra for Cu2p, and from the spectra of as-
prepared catalyst, two main peaks are centred at around 934.7
eV and 954.7 eV for Cu(2p3/2) and Cu(2p1/2) along with the
existence of two characteristic shakeup satellite peaks, which
Fig. 7 N2-Adsorption-desorption and BJH pore size distribution
curves for Cu/RGO catalysts synthesized (a, b) in different
reaction conditions and (c, d) in presence of ultrasound and
ammonia within different Cu wt% loading.
3
0
suggesting that the oxidation state of copper is +2. Whereas,
the XPS spectra for the reduced catalysts as shown in Fig. 8
exhibited two characteristic peaks positioned at around 932.7
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and 952.3 eV for Cu+/0, meanwhile, the disappearance of the difference in the dispersion as well as interactionVibewetAwrtiecleenOntlhinee
shakeup satellite peaks in all the six catalysts is emphasized the RGO support and Cu species. Since, th
DOI: 10 039/C8TA07371F
e wel.1l-dispersed Cu
completely reduction of Cu species surface to the low valence nanoparticles are reduced more easily and also the further
+
0 31
+
+
0
state of Cu or Cu . Nonetheless, the distinguish between Cu
reduction between Cu and Cu may needed a higher reduction
and Cu0 through the XPS analysis is too much difficult. temperature, as can be seen for the Cu/RGO catalyst with high
Therefore, a Cu LMM auger electron spectrum is performed on loading (45 wt%).
the reduced Cu/RGO catalysts to differentiate the concentration
+
0
of Cu or Cu as shown in Fig. 9. From the figure, there are two
overlapping peaks at around 912.3 eV and 916.3 eV in all the 3.3. Catalytic performance
+
six catalysts which assigned to the kinetic energy of Cu and
The reduced Cu/RGO catalysts were subjected to
0
Cu , respectively. The estimated deconvolution results
industrially relevant DMO hydrogenation reaction. Fig. 10
+
+
0
demonstrated that the intensity of Cu /(Cu +Cu ) is increased
within Cu loading from 26.3% for 10 wt% to reach a maximum
value of 35% for the Cu/RGO catalyst within 25 wt% of Cu
loading and then decreased to 26% for Cu/RGO (45 wt%).
While, the Cu/RGO catalyst synthesized using only ultrasound
displayed a low value of 27% compared with 28.3 % for the
o
shows the effect of various reaction temperatures (200-240 C)
on the DMO conversion using Cu/RGO catalysts. Fig. 10a
shows the DMO conversion ratio for the catalysts synthesized
in different reaction conditions, and from the figure, it can be
clearly seen that the Cu/RGO catalyst synthesized in presence
of ultrasound and ammonia exhibited the highest DMO
4
catalyst synthesized in presence of NH OH only. Noteworthy,
conversion ratio at all reaction temperatures below 240 o
C
the shift in the peaks to the lower binding energy after reduced
as shown from the high resolution XPS to Cu, O and C (Fig.
S3) are reflected the electron transfer between the RGO NSs
and Cu species.
compared with the other two catalysts (Cu/RGO (US) and
Cu/RGO (NH OH), meanwhile, all of the three catalysts
achieved 100 % of DMO conversion at 240 C. Whereas, Fig.
0b shows the relation between the DMO conversion ratio and
4
o
1
the Cu/RGO catalysts synthesized within different Cu loading.
From the figure, the Cu/RGO catalyst within high Cu loading
(
45wt%) displayed the highest DMO conversion ratio at
o
o
temperatures below 230 C, but at 230 and 240 C of reaction
temperature, the three catalysts reached the maximum
conversion of 100 %.
Fig. 11 a, b shows the selectivity ratio of MG and ETOH
at different reaction temperatures. From the figure, the
Cu/RGO(US/NH4OH) catalyst exhibited the highest MG
o
selectivity of 98.8 % at 210 C, meanwhile, increasing the
o
reaction temperature to 240 C resulted in increasing the ETOH
selectivity to 89%. However, the other two catalysts ((Cu/RGO
(
4
US) and Cu/RGO (NH OH) showed less MG and ETOH
selectivity compared with the catalyst synthesized using both of
US and NH4OH. On the other hand, the effect of Cu loading on
MG and ETOH selectivity is also studied as shown in Fig. 11 c,
d. From the figure, the Cu/RGO (45 wt%) catalyst displayed the
o
highest ETOH selectivity of 94% at 240 C. Remarkably, no
any side-products or negligible values were obtained in our
reaction such as methyl ether, methyl formate, methyl acetate
or dimethyl carbonate, which reflected the highly efficiency of
the catalyst. The turnover frequency (TOF) values for the six
prepared catalysts, which defined as the number of converted
DMO molecules in one hour per copper site was calculated at
Fig. 9 Cu-LMM Auger spectra for Cu/RGO catalysts synthesized in
different reaction medium and within different Cu wt% loading.
2
10 °C and the results are presented in Table S1. From the
-1
table, the highest TOF value of 17.1 h is obtained for the
Cu/RGO (US/NH4OH) catalyst. Thus, we can inferred from
this result that the catalyst synthesized in presence of
ultrasound and ammonia made it expose more active sites from
the reduction process of CuO species.
To investigate the reducibility properties of the active
phase in Cu/RGO catalysts, H -TPR characterization was
2
carried out as shown in Fig. S4. From Fig. S4 a, it can be seen
that the catalyst synthesized using only ultrasound
Cu/RGO(US) exhibited a broad peak at higher temperature than
other
two
catalysts
(Cu/RGO
(NH
4
OH
and
Cu/RGO(US/NH
4
OH)), which may reflects the strong
interaction of Cu with RGO support. While, two overlapped
peaks are observed for the different Cu loading catalysts, which
2
+
+
+
could be attributed to the gradually reduction of Cu to Cu
+
0
and Cu to Cu , and further confirming the co-existence of Cu
0
and Cu in the catalysts. While, from the TPR profile in Fig.
S4b for the Cu/RGO catalysts within different copper loading,
it can be clearly seen that the increase in the Cu loading from
1
0 wt% to 25 wt% ad 45 wt%, caused a shift in the TPR peaks
to the higher temperature, which might be attributed to the
6
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DOI: 10.1039/C8TA07371F
Fig. 11 MG and ETOH selectivity ratio at different reaction
temperatures of Cu/RGO catalysts synthesized in (a, b) different
reaction condition and (c, d) synthesized in presence of ultrasound and
ammonia within different Cu wt % loading.
Nonetheless, and in absence of noble metals, Wen et al.
found that the hydroxyapatite as support to Cu catalyst can
produce high MG selectivity because the abundant amount of
OH group in the support surface and the phosphate group could
Fig. 10 DMO conversion ratio of Cu/RGO catalysts synthesized
(
+
34
a) in different reaction condition and (b) synthesized in presence stabilize the Cu and enhance the MG selectivity. Recently,
of ultrasound and ammonia within different Cu wt% loading.
Zhao et al. have used mesoporous SiO
2
/Cu catalyst in DMO
hydrogenation reaction and their study concluded that the 53%
+
+
0
of Cu /(Cu +Cu ) which comes from 20% of Cu loading is a
3
3
.4. Discussion
perfect ratio to produce 98.2% selectivity of EG. Therefore,
the noble metals as promoters, the synergistic effect ratio of
In general, MG is the first product in the well-documented
+
0
Cu /Cu and the loading amount of Cu were mainly the key
issues from the most of the reported works in order to obtain
high MG selectivity via the hydrogenation reaction of DMO.
DMO hydrogenation reaction (MG→EG→ETOH), however,
it's difficult to suppress the subsequent hydrogenation steps to
produce EG and ETOH due to the thermodynamic constant of
the first step (MG formation) is much lower than the
subsequent two hydrogenation steps in the reaction. Besides,
the earlier studies have been reported that a high selectivity of
EG and ETOH is achieved using Cu/SiO
the second step in the reaction of EG production has been
scaled up in the large industrial application successfully,
however, noble metals elements such as Ag, Ru and Au is still
a necessary step to obtain high MG selectivity. Furthermore, it
has been concluded that the DMO hydrogenation reaction
occurs on the copper surface and Cu could function as the
primary active site, while, Cu species acts as electrophilic or
Lewis acid sites for polarization the carbonyl bond (C=O)
through the electron lone pair in oxygen and therefore,
promoting the reactivity of the ester group in the reaction.
Thus, increasing the concentration of the Cu as the main
active phase leads to deep hydrogenation of DMO to EG and
ETOH, however, the presence of more active sites of Cu in
In this study and in light of the obtained above results, it
can be inferred that the catalysts synthesis condition has a
significant impact of on the synthesized catalysts properties and
9
2
catalyst and already
+
0
the Cu /Cu ratio and consequently the catalytic performance
DMO conversion and product selectivity). Noteworthy, for the
(
3
2
selectivity part, our results are consistent-well with the reported
previously studies in which the high MG selectivity is obtained
+
0
when Cu /Cu is reached its maximum value. More precisely,
from the obtained Cu LMM auger electron spectrum results, the
0
4
Cu/RGO (US/NH OH) catalyst within 25 wt% of Cu loading
+
+
+
0
exhibited the highest Cu /(Cu +Cu ) value of 35% as well as
the highest MG selectivity value of 98.8% as shown in Fig. 11c.
4
However, the Cu/RGO (US/NH OH) catalyst within 45 wt% of
3
3,
0
0
+
Cu loading and the highest Cu /(Cu +Cu ) ratio displayed high
ETOH selectivity of 94% when the reaction temperature is
0
o
increased to 240 C, as shown in Fig 11d. Whereas, for the
+
DMO conversion part, it can be also seen the great impact
between the synthesis condition, reaction temperature and
activity (Fig. 10). The catalysts synthesized in presence of US
and NH4OH displayed a high DMO conversion than other
catalysts synthesized in presence of one of them at reaction
the catalyst acts to stabilize the intermediate product and
terminate the selectivity to MG. Chen et al. and Liu et al.
have found that the 36% of Cu /(Cu +Cu ) is the ideal ratio to
obtain high MG selectivity but in presence of Ag as
promoter.4 Also, Fan et al. have developed a novel catalyst
3
4
+
+
0
,13
o
temperature below 240 C, and this may be attributed to the
1
2
2 2
of Ru-NH -SiO to obtain high MG selectivity.
high dispersion properties and large pore diameter of Cu
nanoparticles as mentioned above. Besides, the Cu loading on
Cu/RGO catalysts was also found to have an effect on the
DMO conversion, and the activity is clearly increased within
the increasing of Cu loading from 10 to 25 and reached the
maximum for the Cu/RGO catalyst within 45 wt% at reaction
o
temperature below 230 C, and the reason here mostly due to
the increasing in the amount as well as ratio of active Cu0
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Journal of Materials Chemistry A
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ARTICLE
Journal Name
0
species. Since, Gong et al. have reported that Cu could be the study, noble metal-free Cu/RGO catalyst syntheVsiiezweAdrticbleyOnoliuner
DO 10.1 9/C8TA07371F
active site and primarily responsible about the activity in DMO novel sonochemical method exhibited outI:stand03ing catalytic
3
3
hydrogenation reaction. So, that concept is matched well with performance in DMO hydrogenation reaction and displayed
0
the obtained results here for the high Cu ratio as well as high high MG selectivity and stability for at least 300 h. Moreover,
DMO conversion ratio for the Cu/RGO catalyst synthesized the employed sonochemical method is a green and performed in
within 45 wt%.
a very short time without the need for any surfactant or
On the other hand, RGO NSs as support is enhanced the additional reducing agent as mostly utilized in other
electron transfer to the Cu species and worked as reducing conventional approaches, which confirmed the high efficiency
0
+
agent, thus, increased the active sites of Cu and Cu , which of our method for the synthesis of highly efficient
caused the improvement in the catalytic performance. In heterogeneous catalyst.
addition, based on the reported work by Vinodgopal et al., the
ultrasonocation process is producing defects as well as restoring
graphitic structure,35 thereby, the abundance numbers of defect
sites, dislocations and the defective edges which existed in the
RGO NSs as can be seen in the TEM images for the spent
catalyst in supporting information are anticipated to boost the
catalytic performance of Cu/RGO catalyst and also keep the
high stability.3 Also, compared with the obtained thick layers
6,37
of RGO NSs for the catalysts synthesized in absence of
ultrasound (Cu/RGO (NH4OH), the acoustic cavitation is
produced the so-called shear forces, which are sufficient to
overcome the van der Waals forces between the sheets of
3
5
graphene, therefore,
prevent their re-aggregation, and
consequently produced the obtained very thin and highly
dispersed Cu/RGO catalysts (as can be seen in the TEM images
for the catalysts synthesized in presence of ultrasound), thereby,
improved the catalytic properties.
The long term stability is regarded as a critical parameter
to the industrial application; therefore, our produced catalyst is
subjected to the stability test for at least 300 h as shown in Fig.
S6. From the figure, we can see clearly the excellent stability
properties of our catalyst in terms of DMO conversion as well
o
as MG selectivity at 210 C. More precisely, still the MG
selectivity and DMO conversion ratio are higher than 90% after
3
00 h of reaction on stream, which reflected the great potential
application of our catalyst in the industry according to its high
activity and stability in DMO hydrogenation reaction. The main
reason for this kind of superior stability is may be ascribed to
the electron transfer process between the RGO as a support and
Cu species which keep the activity of the interface and thus
improve the stability and also the exceptional high dispersion
properties. Fig. 12 shows the TEM images for the spent
Cu/RGO catalysts with different loading. From the figure,
although there is a grain growth in the particles size is
observed, however, still the catalysts within 10 and 25 wt% of
Cu loading have an excellent dispersion and without any
aggregation after the long term of reaction. The inset of Fig.
Fig. 12 TEM images for the spent Cu/RGO catalysts
synthesized in presence of ultrasound and ammonia (a, b) 10
wt%, (c, d) 25 wt% and (e, f) 45 wt%.
1
2b shows the HRTEM images of the catalyst within clear
crystal planes (111) of Cu metals, reflecting the highly
crystalline structure and stability. Additional TEM and HRTEM
images for the spent Cu/RGO catalysts are provided in the
supporting information (Fig. S6).
On the other hand, we felt necessary and very important
to compare the catalytic activity of our produced Cu/RGO
(
4
US/NH OH) catalyst within 25 wt% of Cu loading and other
catalysts synthesized using various methods such as
impregnation, galvanic deposition and hydrothermal methods,
and within different supports materials such as CNTs, SiO
as shown in Table. 1, and the detailed reaction conditions of
/DMO ratio and temp of the employed catalysts in the
2
, etc,
H
2
comparison of the catalytic activity are supported in Table S2.
Interestingly, we can understand clearly from the table that the
noble metals such as Ag and Ru were necessary as promoters to
1
,4,12,13, 38,32,39
obtain high MG selectivity.
However, in this
8
| J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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ARTICLE
1
. J. Zhou, X. Duana, L. Ye, J. Zheng, M.M. LVi,iewS.AErt.icTlesOannlinge,
Y. Yuan, Applied Catalysis A: General., 2015, 505,
Table 1 Catalytic performance of different catalysts synthesized by various approaches
DOI: 10.1039/C8TA07371F
3
44–353.
DMO
Conversion
%
MG
Selectivity
%
2
3
. Y. Song, J. Zhang, J. Lv, Y. Zhao, X. Ma, Ind. Eng.
Type of Catalyst
Synthesis method
Stability test
Ref
Chem. Res., 2015, 54 , 9699–9707.
. Y. Zhao, Y. Zhang, Y. Wang, J. Zhang, Y. Xu, S. Wang,
X. Ma, Applied Catalysis A, General., 2017, 539, 59–
Co-Impregnation
method
Ag-Ni/SBA-15
Ag-CuOx
82.9
>96
99.4
26.7
>96
96.3
96
140 h
1
6
9.
Galvanic deposition
Not tested
Not tested
1100 h
4
4. Y. Chen, L. Han, J. Zhu, P. Chen, S. Fan, G. Zhao, Y. Liu,
Y. Lu, Catalysis Comm ., 2017,96, 58–62.
5. Z. Chen, J. Zhang, M. Abbas, Y.Y. Xue, J. Q. Sun, K.F.
Liu, J. Chen, Ind. Eng. Chem. Res. 2017,56 (33), 9285-
9292.
. C. Wen, Y. Cui, W. Dai, S. Xie and K. Fan, Chem.
Commun., 2013,49, 5195.
Impregnation
method
Ru-NH
2
-SiO
2
87.3
98.6
>97
12
13
32
Ag/Cu/SiO
2
Galvanic deposition
6
Impregnation
method
Ag/CNTs
200 h
7
. Y. Zhao, S. Li, Y. Wang, B. Shan, J. Zhang, S. Wang, X.
Impregnation
method
Ma, Chemical Engineering Journal., 2017, 313, 759–
Ag-B
2
O
3
/SiO
2
98.9
>90
100
97.2
94
260 h
120 h
300 h
38
39
7
68.
8
. J. Ding, Y. Liu, J. Zhang, M. Dong, Y. Wang, W. He, X.
Han, K. Liu, Z. Jiang, J. Chen, Molecular Catalysis.,
2017,438, 93–102.
Cu/SiO
2
Sonochemical
Sonochemical
Cu/RGO (25
wt%)
This
work
98.8
9
1
. J. Zheng, H. Lin, X. Zheng, X. Duan, Y. Yuan, Catalysis
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4
. Conclusions
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013,297, 110.
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In summary, eco-friendly and surfactant-free in-situ
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2. H. Fan, J. Tan, Y. Zhu, H. Zheng, Y. Li, Journal of
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sonochemcial reaction was designed to synthesize highly
dispersed and morphology controlled Cu nanoparticles-
wrapped on RGO NSs catalysts. The prepared catalysts were
employed for the first time in DMO hydrogenation reaction
without the pre-calcination step and displayed remarkable
catalytic activity. The catalyst synthesized in presence of
ultrasound and ammonia within 25 wt% of Cu loading showed
high MG selectivity of 98.8% at 210ºC, while, the highest
ETOH selectivity value of 94% is produced by the catalyst
synthesized within 45 wt% of Cu loading at 240 ºC. Moreover,
both of catalysts exhibited superior stability for at least 300 h,
which reflecting their promising potential industrial application.
This could be attributed to the outstanding dispersion of Cu
caused by the applied of the high intensity ultrasound, the
synergistic and proper ratio of Cu /(Cu +Cu ) and the
exceptional properties of RGO as catalysts support. The
developed highly efficient and convenient sonochemical
approach holds great promise for the clean synthesis of various
active heterogeneous catalysts; meanwhile, the Cu/RGO
catalyst could be used in other various hydrogenation reactions.
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The authors have no conflicts of interest
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Acknowledgements
This work was supported by NSFC (Natural Science
24. Y. Wang, H. Li, J. Zhang, X. Yan, Z. Chen,
Phys.Chem.Chem.Phys. 18(2016) 615.
Foundation of China project (21673272) and
（21373254） and
the Postdoctoral fellowship project of President's International
Fellowship Initiative (PIFI) by Chinese Academy of Sciences,
China.
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Journal of Materials Chemistry A
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DOI: 10.1039/C8TA07371F
An eco-friendly and In-situ sonochemical approach is designed for the synthesis of morphology
controlled Cu nanoparticles wrapped on RGO NHs catalysts and their superior stability and catalytic
application for the synthesis of Methyl Glycolate and Ethanol from Dimethyl oxalate hydrogenation
reaction.