Nanoparticles of Cu-Co alloy derived from layered double hydroxides and their catalytic performance for higher alcohol synthesis from syngas

Article abstract of DOI:10.1039/c5ra05190h

A series of layered double hydroxides (LDHs) with different Cu/Co ratios were prepared according to the co-precipitation method and used as catalyst precursors for higher alcohol synthesis. The prepared samples were characterized by XRD, TPR, SEM, TEM and BET techniques. After calcination, the LDHs were transformed into a mixture of CuO, Co₃O₄, CuCo₂O₄ and alumina, and these oxides were mixed uniformly with sizes of several nanometers. For the sample with a ratio of Cu/Co = 1/2, the copper and cobalt species are mainly in the CuCo₂O₄ phase. In the reduction process, the superior mixed copper and cobalt species in nanoscale were reduced to Cu-Co alloy, which was confirmed by the XRD and TEM results. The prepared bimetallic catalysts showed high activity, good stability and very high selectivity to C₂₊ alcohols. With the best catalyst, CO conversion of 51.8%, selectivity to alcohols of 45.8% and 94.3 wt% of C₂₊OH in the total alcohols were obtained at 250 °C, 3 MPa and GHSV of 3900 mL (g_cat h)^-1.

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Nanoparticles of Cu-Co alloy derived from layered double
hydroxides and their catalytic performance for higher alcohols
synthesis from syngas
Received 00th January 20xx,
Accepted 00th January 20xx
Ang Cao a, b, Guilong Liu a, b, Yizhi Yue a, b, Lihong Zhanga, b, Yuan Liu* a, b
DOI: 10.1039/x0xx00000x
A series of layered double hydroxides (LDHs) with different Cu/Co ratio were prepared according to the co-
precipitation method and used as catalyst precursors for higher alcohols synthesis. The prepared samples were
characterized by XRD, TPR, SEM, TEM and BET techniques. After calcination, LDHs were transformed into a
mixture of CuO, Co3O4, CuCo2O4 and alumina, and these oxides were mixed uniformly with size in several
nanometers. For the sample with ratio of Cu/Co = 1/2, copper and cobalt species are mainly in CuCo2O4 phase. In the
reduction process, the superior mixed copper and cobalt species in nanoscale were reduced to Cu-Co alloy, which was
confirmed by XRD and TEM results. The prepared bimetallic catalysts showed high activity, good stability and much
high selectivity to C2+ alcohols. Over the best catalyst, CO conversion of 51.8%, selectivity to alcohols of 45.8% and
94.3 wt. % of C2+OH in the total alcohols were obtained at 250ºC, 3MPa and GHSV of 3900 mL (gcat h)-1.
www.rsc.org/
industrialized for tens of years with methanol selectivity
close to 100%. Therefore, only the selectivity to C2+
1.Introduction
With the dwindling of petroleum resource and the
increasing consumption for energy, developing alternative
fuels for the sustainable world economics are urgently
alcohols is high enough, excluding separation cost, HAS
may be of value for industrialization.
Catalysts for HAS from syngas can be categorized to
four types, including the modified methanol catalysts, Rhꢀ
based catalysts, Moꢀbased catalysts and modified Fisherꢀ
Tropsch catalysts. Modified methanol synthesis catalysts
2
needed.1, Higher alcohols synthesis (HAS) via syngas
from coal, natural gas or renewable biomass has attracted a
considerable amount of attention due to the potential
application of higher alcohols as motor fuels, gasoline
blends additives and other intermediates for chemical feed
6
produce mainly methanol . Rhꢀbased catalysts possess a
comparatively high selectivity to ethanol and good activity
7, 8, while the price of Rh is too high. Moꢀbased catalysts
exhibit excellent sulfur resistance, however, they must be
operated at high pressures and temperatures, and usually a
long activity induction period before the reaction is
stocks 3ꢀ5
.
However, the existing technologies for HAS are still
in the laboratory stage, one of the critical problem is that
the selectivity to C2+ alcohols is too low. Over most of the
catalysts reported, methanol is the main product instead of
higher alcohols. The price of each C2+ alcohols, including
ethanol, propyl alcohol and butyl alcohol, is more than
twice of that of methanol, and production of methanol is
required 9, 10
.
The modified FisherꢀTropsch catalysts include CuꢀFe
and CuꢀCo based catalysts, of which CuꢀFe catalysts tend
to initiate water gas shift reaction (WGSR) and thus
generate more CO2. Comparatively, CuꢀCo based catalysts
show good performance for HAS at mild reaction
conditions, and become one of the most promising
a.Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China. Email: yuanliu@tju.edu.cn. Tel: +86 022 87401675.
b.Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of
Chemical Engineering, Tianjin University, Tianjin 300072, China.
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catalysts for practical application.
cations, respectively. Anꢀ is the interlayer anion, to make
For CuꢀCo based catalysts, copper acts as the active electrical neutrality with the cations. In LDHs, the metal
site for the nonꢀdissociative adsorption of CO and the cations of M2+ and M3+ are uniformly distributed at atomic
dissociative chemisorption of H2; while cobalt is level in the bruciteꢀlike layers. A character of LDHs is that
responsible for the dissociative activation of CO and the uniform distribution of the metal species would be
carbon chain growth. Therefore, the synergistic working of maintained in the resultant mixed metal oxides after
the copper and cobalt sites can promote the generation of calcination although the LDHs structure would be
higher alcohols11ꢀ13. It is acknowledged that the formation destroyed, which is owing to the topological effect of
of CuꢀCo alloy is beneficial for the synergistic catalyzing. LDHs27, 28. The uniform distribution of the metal species in
Hence the formation of CuꢀCo alloy is important for HAS, the mixed metal oxides and the topological effect of LDHs
and the formation of CuꢀCo alloy can increase the are beneficial and crucial for preparing supported nano
selectivity to C2+ alcohols14ꢀ17
In order to form CuꢀCo alloy, CuCoO2
CuCo2O4 14, 19 were used as the precursors, in which cobalt the precursors can be found. For instance, Li et al.
.
metal alloy
Studies on bimetallic catalysts prepared via LDHs as
18
and
24
and copper ions are uniformly distributed at the atomic prepared CoꢀNi based catalysts by calcinating and
level. Consequently, the formation of CuꢀCo alloy should reducing LDHs precursors, which exhibited high activity
be favored in the reduction process. However supported and good stability for ethanol steam reforming. Yang et al.
29
catalysts generally are prepared by impregnation method
reported that uniform CoꢀFe alloy nanoparticles with a
followed by drying and calcination. During the narrow size distribution were obtained by reducing the
impregnation process, making the metal species being LDHs precursors which contain Mg, Co and Fe ions.
uniform and highly dispersed on the support is vital to the However, to the best of our knowledge, using the LDHs as
performance of catalyst. There are many influencing the precursor to prepare supported nanoparticles of CuꢀCo
factors, such as the interaction between the metal ions with alloy for HAS has not been reported.
the support, the migration and aggregation of the metal
In the present investigation, CuꢀCo alloy
species during the drying and calcination processes. Thus, nanoparticles supported on Al2O3 were prepared by using
in the resulting supported CuꢀCo based catalysts, apart LDHs as the precursor. The resulted catalysts showed good
from CuCoO2 and CuCo2O4, CuO and Co3O4 would be catalytic performance for HAS from syngas, especially
inevitably formed. After reduction, mono metal particles of exhibited high selectivity to C2+ alcohols.
copper and cobalt derived from CuO and Co3O4 would be
2.Experimental
generated, reducing the selectivity to C2+ alcohols by
2.1Materials
producing methanol and hydrocarbons, respectively.
Analytical grade chemicals of Cu(NO3)2•3H2O,
Layered double hydroxides (LDHs), a class of
Co(NO3)2•6H2O, Al(NO3)3•9H2O, NaOH and Na2CO3
synthetic
twoꢀdimensional
nanostructured
layered
were purchased from Aladdin and used without further
purification. Deionized water was used in all the
experimental processes.
materials 20ꢀ22, have recently received significant attentions
owing to their promising applications as heterogeneous
23ꢀ25
catalysts
and catalyst supports 26. LDHs can be
2.2Preparation of Cu-Co catalysts
expressed by a typical formula [M2+
M
1ꢀx
(OH)2]x+ (Anꢀ
3+
x
A
series of (CuxCoy)2AlꢀLDHs (([Cu2+]
+
)
x/n mH2O, where M2+ and M3+ are divalent and trivalent
2 | J. Name., 2012, 00, 1-3
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[Co2+])/[Al3+] = 2 in molar) with Cu/Co molar ratio of 1:0, (EDX) for the determination of metal composition and
1:1, 1:2, 1:3 and 0:1 was prepared by a coꢀprecipitation elemental mapping were obtained on a JEOL JEMꢀ2100F
method.
Cu(NO3)2•3H2O,
Co(NO3)2•6H2O
and microscope
fieldꢀemission
transmission
electron
Al(NO3)3•9H2O were dissolved in deionized water at [Cu2+] microscope. After ultrasonic dispersion of the catalysts in
+ [Co2+] + [Al3+] = 1.0 M. NaOH and Na2CO3 were absolute ethanol, the wellꢀdispersed samples were dropped
dissolved in deionized water with [NaOH] = 1.5 M and and dried on a Mo grid with a layer of holey carbon film.
[CO32ꢀ] = 2[Al3+]. The two solutions were simultaneously
Temperature programmed reduction (TPR) tests were
added into deionized water under vigorous stirring, carried out on a fixed bed microꢀreactor in order to study
maintaining pH = 9.5 ± 0.1. As the precipitation was the reducibility of the metal species in the catalysts. In
completed, the slurry was aged at 80 °C for 12 h, and then each run, 50 mg of the catalyst was loaded into a quartz
filtered, washed with distilled water and dried at 70 °C for tube reactor and pretreated with 5% H2/Ar to remove the
12 h. The thus prepared LDHs were calcined at 500 °C for air in the reactor at room temperature. Then, the catalyst
3 h, denoted as (CuxCoy)2AlꢀCLDHs, and then reduced at was heated from room temperature to 800 °C at a heating
450 °C for 3 h in hydrogen. The reduced catalysts were rate of 10 °Cminꢀ1 in the presence of 5% H2/Ar flow at a
named as (CuxCoy)2AlꢀRed.
flow rate of 48 mL minꢀ1.
2.3Catalyst characterization
2.4 Catalyst characterization
The higher alcohols synthesis tests were conducted in
The phase analyses of the catalysts were determined
by the XꢀRay Diffraction (XRD) technique which was a fixedꢀbed, stainless flow microꢀreactor with a length and
recorded on an X'Pert Pro instrument, a Co Kα radiation inner diameter of 300 nm and 8 nm, respectively. 800 mg
source was used with an accelerating voltage of 40 kV and of catalyst with 40ꢀ60 mesh grain size was diluted with
electric current of 40 mA. The spectrum was collected at a 3200mg quartz sand with the same size and then loaded
scanning speed of 5 ° minꢀ1.
into the reactor, the mass ratio of the catalysts and quartz
Nitrogen adsorption and desorption isotherms were sand was 1:4. All catalysts were activated onꢀline under a
performed on a Trwastar 3000 micromeritics apparatus at ꢀ flow of H2 at 450 °C for 3 h at a rate of 2 °C minꢀ1.
196°C. The specific surface areas were calculated based on Subsequently, the reactor was cooled to room temperature,
the BET method and the pore size distributions were and the pressure was increased to 3 MPa by feeding the
calculated from the adsorption branch of the isotherms syngas mixture (H2/CO/N2 = 8:4:1, where N2 was used as
using the BJH model. All samples were outgassed under the internal standard gas for analyzing the reacted gases).
vacuum at 300°C for 4 h prior to analysis.
Scanning electron microscopy
The gas hourly space velocity was set at 3900 mL (gcath)ꢀ1.
(SEM) At each reaction temperature form 250 to 290 °C, the
characterizations were performed on a Hitachi Sꢀ4800 reaction was maintained for 1.5 h to obtain steadyꢀstate
fieldꢀemission scanning electron microscope to observe the activity and selectivity; except at 240 °C the reaction was
morphology of the samples, which were treated with Au conducted for 3 h. A gas chromatograph with two packed
sputtered first.
columns was used to analyze the products. The gas
Transmission electron microscopy (TEM) pictures products of CO, H2, CH4, CO2 and N2 were separated
and high angle annular dark field scanning transmission online using a TDXꢀ01 packed column (2 m) connected to
electron microscopy (HAADFꢀSTEM) line scans a TCD detector. After separation by condensation, liquid
combined with energy dispersive Xꢀray spectroscopy products and hydrocarbons were analyzed offꢀline using a
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Porapak Q column (3 m) connected to a FID detector.
CO conversion (XCO) and the product selectivity (Si)
were calculated according to the following
CO − CO
out
in
equations:
X
=
× 100%
CO
CO
in
nC
i
S =
× 100%
i
nC
∑
i
Where COin, COout is the moles of CO in the feedꢀ
gases and offꢀgases, respectively; n, Ci represents the
number of carbon atoms in the molecule and moles of a
carbonꢀcontaining product, respectively.
Fig. 1 XRD patterns of (I) (CuxCoy)2AlꢀLDHs, (II) (CuxCoy)2Alꢀ
CLDHs and (III) (CuxCoy)2AlꢀRed with x/y ratios of (a) 1:0, (b) 1:1,
(c) 1:2, (d) 1:3 (e) 0:1. Crystalline phases: (●) Co3O4/CuCo2O4, (♥)
CuO, (♣) Co, (♠) Cu, (♦) CuꢀCo alloy; (III') the variation of Cu/Co
molar ratio vs. lattice parameter a.
3.Results and discussion
3.1XRD
The XRD patterns of asꢀsynthesized (CuxCoy)2Alꢀ
LDHs are shown in Fig. 1(I). All samples show sharp and
symmetric diffractions at 2 θ values of 13.5, 27.2 and 40.5
º, ascribed to the crystalline planes of (003), (006) and
(009) for LDHs, respectively, indicating wellꢀcrystallized
hydrotalcite structures. No any detectable impurity phases
can be observed, indicating the high purity of LDHs
structure in the samples.
The basic spacing (d003), varies between 0.746 and
2ꢀ
0.752, which is close to the values for CO3 containing
LDHs with M2+/Al3+ ratio of about ~ 2 32. Thus, CO32ꢀ was
successfully introduced into the space between the
hydrotalcite laminates.
Fig. 1 (II) shows the XRD patterns of (CuxCoy)2Alꢀ
CLDHs. It can be seen that the diffraction peaks
corresponding to LDHs disappear, and the diffraction
peaks of metal oxides appear, suggesting that the LDHs
structures are completely destroyed and transformed into
metal oxides after calcination. In addition, the diffraction
peaks corresponding to Al2O3 cannot be observed,
indicating that Al2O3 is in amorphous state.
Particularly, the XRD patterns show a variation trend
correlated to the chemical compositions. With the increase
content of copper, the diffraction peaks moves to a little
higher 2 θ values (Fig. 1(I) inset). This is attributed to the
distortion of copper ions in LDHs as pointed by Khan et al.
30. Similarly, Liu et al. also pointed out that Cu2+ ions in
LDHs showed JahnꢀTeller effect that favored the formation
of distorted octahedral structure 31, which lead to the shift
of the diffraction peaks. Compared with copper ion, cobalt
ion is easier to form LDHs structure, thus the two
diffraction peaks of (110) and (113) at 2 θ values of around
71 and 74 º become sharper and stronger with cobalt
content increasing.
For the Cu2AlꢀCLDHs, only a peak attributed to CuO
crystalline phase at 2θ = 41.4 º exists, and the peak is
broad, meaning a small size of the CuO nanoparticles.
As for the cobalt containing samples, the diffraction
peaks corresponding to CuO phase can hardly be seen,
which is attributed to the following two reasons. One is
that the copper and cobalt species are combined to form
CuCo2O4 during calcination, which is supported by the
presence of the characteristic diffraction peaks of CuCo2O4.
The other is that the left copper species in CuO phase are
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highly dispersed or in small quantity. As the atomic ratio of observed, which can be clearly seen from the inset picture
34
Cu/Co equals to 0.5, which is the stoichiometric ratio of in Fig. 1 (III). Su et al. attributed the diffraction peaks
Cu/Co in CuCo2O4, copper should be mainly in CuCo2O4. between pure metal Co and Cu to the bimetallic CoꢀCu
As further increasing cobalt content, CuCo2O4 and Co3O4 alloy. Volkova et al. 18 demonstrated that the slight increase
are the major phases in the catalyst. This is revealed by the in lattice spacing for copper and a small decrease in lattice
slight shift of the lattice parameter from 8.105 Å of pure spacing for cobalt are the characteristics of the CoꢀCu alloy.
CuCo2O4 to 8.101 Å. Both CuCo2O4 and Co3O4 belong to Hence, CuꢀCo alloy was formed in (CuxCoy)2AlꢀRed.
spinel structure, while the lattice parameter of Co3O4 is
As shown in Fig. 1 (III'), the metal lattice parameter
slightly smaller due to the smaller size of cobalt ion. of a for (CuxCoy)2AlꢀRed is smaller than that of Cu0 (3.613
Finally, Co3O4 becomes the only oxide with spinel phase Å), larger than that of Co0 (3.548 Å), and decrease linearly
for Co2AlꢀCLDHs catalyst.
with the increase of cobalt content. This is in accordance
In summary, copper and cobalt exists in CuCo2O4 with that the size of cobalt atom is smaller than that of
35ꢀ37
accompanied with CuO or Co3O4 in the calcined catalysts, copper, supporting the formation of Cu ꢀ Co alloy
.
35
and copper and cobalt existed mainly in CuCo2O4 for Vegard’s law
(Cu1Co2)2AlꢀCLDHs.
pointed out that independent of other
factors, the microscopic crystal structures of alloys depend
In addition, all of the diffraction peaks are broad, on the atomic sizes and relative concentrations of the
suggesting that the crystal grains are small. The metal constituent. For CuꢀCo alloy, the lattice parameter a should
oxides in the calcined samples came from LDHs, where decrease linearly with increasing Co content due to the
the metal ions were uniformly mixed at atomic level, so smaller size of cobalt than that of copper, and vice versa,
the metal oxides should be uniformly mixed. In other the linearly variation is an indication for the CuꢀCo alloy
words, CuCo2O4, CuO and Co3O4 were well mixed and formation.
highly dispersed on the amorphous Al2O3 matrix. The
Although some CuO and Co3O4 were formed
nanoparticles of the oxides should be in interaction, which inevitably after calcination of LDHs, considering that the
further limited the migration and aggregation of those metal ions are uniformly distributed in an atomic level in
oxides. This means that the resulted mixed oxide possesses the layers of LDHs, all the CuO, Co3O4 and CuCo2O4 with
high antiꢀsintering ability, which agrees with the results in small and uniform particle sizes were mixed evenly on the
the literature 33, where ZnO/ZnAl2O4 nanocomposite was amorphous Al2O3 matrix. Thus the interaction between
made by using LDHs as the precursor and exhibited high them should be enhanced, and consequently, CuꢀCo alloy
thermal stability.
rather than separate copper and cobalt are formed after
Fig. 1 (III) shows the XRD results of reduced reduction.
catalysts. For Co2AlꢀRed and Cu2AlꢀRed, diffraction peaks
corresponding to metal cobalt and copper can be seen
clearly. No diffraction peak related to alumina can be
detected, showing alumina is in amorphous state.
3.2TPR
For (CuxCoy)2AlꢀRed containing both copper and
cobalt elements, diffraction peaks corresponding to copper
and cobalt containing oxides disappear, while peaks
between the diffraction peaks of pure metal Cu and Co are
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reduced at much lower temperature, seeing the reduction
profile for Cu2AlꢀCLDHs. Secondly, the area of this
reduction peak increases with the content increase of
cobalt in the catalyst. As compare to Co2AlꢀCLDHs, the
reduction temperature of this peak for cobalt oxide is
obviously lower, which is ascribed to the formation of
metal CuꢀCo at the lower reduction temperatures. It is
known that metal particles can activate hydrogen and
catalyze the reduction of metal ions nearby 43, 44
.
Fig.
2
TPR patterns of catalysts of (a) Cu2AlꢀCLDHs, (b)
For (CuxCoy)2AlꢀCLDHs, the reduction peaks in
temperature range of 150 to 250 °C are corresponding to
the formation of bimetal CuꢀCo. Compares to the reduction
profiles of Co2AlꢀCLDHs and Cu2AlꢀCLDHs (Fig. 2 a and
e), the reduction temperatures of (CuxCoy)2AlꢀCLDHs (Fig.
2 b, c and d) are obviously lower. The difference between
them is that (CuxCoy)2AlꢀCLDHs contains CuCo2O4, and
CuCo2O4 is the main phase for (CuxCoy)2AlꢀCLDHs as the
above XRD results shown. Thus, the reduction peaks in the
temperature range of 150 to 250 °C are attributed to the
reduction of CuCo2O4 to metal copper and metal cobalt.
The XRD results indicate that, besides CuCo2O4,
there are Co3O4 and CuO in (CuxCoy)2AlꢀCLDHs. The
reduction of the CuO can be included in the reduction
peaks at 150 to 250 °C, for that the bimetal of CuꢀCo
reduced from CuCo2O4 can catalyze the reduction of
copper ions, resulting into the decrease of reduction
temperature for CuO. The Co3O4 may be partly reduced at
the temperature range of 150 to 250 °C for the cobalt
species weakly interacted with alumina, and partly reduced
at about 450 °C for the cobalt species interacted with
alumina.
(Cu1Co1)2AlꢀCLDHs, (c) (Cu1Co2)2AlꢀCLDHs, (d) (Cu1Co3)2Alꢀ
CLDHs and (e) Co2AlꢀCLDHs.
The TPR profiles of the calcined catalyst are shown in
Fig. 2. For Co2AlꢀCLDHs, two reduction peaks in the
temperature range of 310 ꢀ 410 °C and 600 ꢀ 780 °C are
observed, according to previous reports 14, 38, 39，which
should be attributed to the reduction of Co3O4 to CoO and
CoO to metallic cobalt, respectively. The H2 consumption
ratio of the two peaks is close to 1:3, which supports the
attribution. This result also suggests that cobalt species are
mainly in Co3O4 state, in consistent with XRD results.
40
A.Y. Khodakov et al. stated that for the interaction
between metal oxide and support the smaller the particle is,
the stronger the interaction, and thus the metal oxide with
smaller size is harder to be reduced. Compared with that of
Co3O4 reported by Chu et al. 41, the reduction temperature
of Co3O4 in Co2AlꢀCLDHs is higher, which is likely due to
the smaller size of Co3O4 in Co2AlꢀCLDHs.
For Cu2AlꢀCLDHs, two reduction peaks centering at
around 220 and 300 °C can be observed, corresponding to
the reduction of CuO to metallic Cu, as reported in the
literature 42. The peak at the lower temperature can be
attributed to the reduction of highly dispersed CuO and the
other is ascribed to CuO which in strong interaction with
the Al2O3 support.
To note the reduction profiles of (CuxCoy)2AlꢀCLDHs,
copper and cobalt ions would be reduced to metallic state
simultaneously at 150 to 250 °C. The XRD results and the
properties of the LDHs precursor suggest that copper and
cobalt species in (CuxCoy)2AlꢀCLDHs are uniformly and
highly dispersed in small size (see Tab. 1). The
simultaneously generated metal copper and metal cobalt
For (CuxCoy)2AlꢀCLDHs, the reduction peaks at
about 450 °C should belong to the reduction of cobalt ions.
The reasons are as follows. Firstly, copper ions could be
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2ꢀ
2ꢀ
grains, which are in small size and uniformly mixed, CO3 and water can be divided into two categories. CO3
2ꢀ
should inclined to form CuꢀCo alloy.
ions were located between the sheets of LDHs, thus CO3
leaving would cause slitꢀshaped pores between the sheets;
while water in the state of hydroxyls is in the sheets of
LDHs, thus water leaving would result micro and meso
pores in the sheets. By the way, due to the topological
property of the LDHs, the calcined samples maintain the
morphology, that is, the shape of sheet was preserved. On
the whole, the pores mainly came from the accumulation
of particles, the accumulation of the oxide sheets formed
the slitꢀshaped pores in several nanometer size, and the
accumulation of the sheets constituted the larger particles
in tens nanometer which stacked and formed larger pores.
This means that the surface area mainly came from the
particles, hence no evident capillary condensation is
exhibited in the sorption isotherms of Fig. 3.
3.3BET
Fig. 3 (I) shows the N2 adsorptionꢀdesorption
isotherms and the corresponding pore size distribution of
(CuxCoy)2AlꢀCLDHs. All samples exhibit a typical IV
isotherm with an H3ꢀtype hysteresis loop, suggesting the
presence of mesopores. This result is further confirmed by
the corresponding pore size distribution in Fig. 3 (II).
Fig. 3 N2 adsorptionꢀdesorption curves (I) and pore size distributions
(II) of (a) Cu2AlꢀCLDHs, (b) (Cu1Co1)2AlꢀCLDHs, (c) (Cu1Co2)2Alꢀ
CLDHs, (d) (Cu1Co3)2AlꢀCLDHs and (e) Co2AlꢀCLDHs.
The hierarchical porous structure should play a
favorable role on the higher alcohols synthesis. Ding et al.
46 found that CuꢀFe supported on the bimodal porous SiO2
exhibited good catalytic activity and high selectivity to
C2+OH, which was attributed to the well dispersion of
active metal sites and high diffusion efficiency of products
As can be seen from the pore size distribution, there
are two main pore size distributions in the calcined
catalysts. One is meso pores in size of several nanometers,
and the other in size of tens nanometers. The former is
2ꢀ
proposed to be formed from the left of CO3 and water in
47
inside the bimodal porous structure. Lu et al. found that
LDHs in the calcination process, and the latter came from
the accumulation of oxides particles. From the SEM
images, it is seen the particle size is in tens nanometer, in
accordance for accumulating the pores. The similar view
threeꢀdimensionally ordered macroporous CuꢀFe catalysts
showed high activity and selectivity to C2+OH, because of
the unique ordered porous structure of the catalysts, the
uniformly distributed active components and the synergetic
effect between Cu and χꢀFe5C2.
point has been stated in the literatures 28, 45
.
For more detail, the pores formed from the left of
Table 1 Physical properties and crystal sizes of the catalysts
Samples
BET surface
Area (m2 gꢀ1)
BJH pore
Volume(cm3 gꢀ1)
Pore diameter
(nm)
Crystal Sizea
Crystal Sizeb
(nm)
(nm)
23.2
7.2
Co2AlꢀCLDHs
(Cu1Co3)2AlꢀCLDHs
(Cu1Co2)2AlꢀCLDHs
(Cu1Co1)2AlꢀCLDHs
Cu2AlꢀCLDHs
137
123
106
90
0.47
0.45
0.39
0.33
0.26
3.03
17.8
3.14
4.3
4.8(6.2c)
3.35
8.4
3.80
9.3
5.1
61
4.02
4.7
15.4
a
Crystal sizes of the catalysts after calcination were calculated by Xꢀray diffraction with Scherrer equation.b Crystal sizes of the catalysts
after reduction were calculated by Xꢀray diffraction with Scherrer equation.c Data for the catalyst after 200 hours’ stability test.
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The specific surface areas and the pore structure SEM as shown in Fig. 4. The LDHs precursors display
parameters are shown in Table 1. The BET surface areas smooth and uniform nanocrystals with a narrow particle
ranged in 61ꢀ137 m2 gꢀ1 and the pore volumes in 0.26ꢀ0.47 size distribution of 30 ꢀ 70 nm. Owing to the topological
cm3 gꢀ1. The specific surface area and pore volume effect of hydrotalcite, when the LDHs was calcined and
decreases with the increase of copper content, which may reduced, the same morphology to LDHs was still
be due to the different shape. With increasing cobalt maintained, while the particle sizes became smaller than
content, the shape of particles changed from cubic or that of the precursors, see Fig. 4dꢀ 4f. The similar
rectangle to platelet, see SEM images in Fig. 4aꢀ 4c, and morphology of the samples after calcination and reduction
the platelets have higher specific surface area. The shape with LDHs confirmed the existence of the topological
difference of the morphology is likely due to the Jahnꢀ effect, supporting the above viewpoint that CuO, Co3O4
Teller effect of Cu2+ ions, as pointed in the literature 31
.
and CuCo2O4 would be mixed evenly in the Al2O3 matrix,
as a result, the formation of CuꢀCo alloy are favored after
reduction.
3.4SEM
3.5TEM
TEM pictures are presented in Fig. 5(a).
A
representative high resolution TEM image is shown in the
inset picture in Fig. 5 (a), and the lattice space of 2.08 Å is
corresponding to the (110) crystal plane of CuꢀCo alloy,
suggesting the formation of CuꢀCo alloy, which is
consistent with the XRD results in Fig. 1 (III).
It can be seen from Fig. 5 (a) that the CuꢀCo alloy
nanoparticles are evenly distributed throughout the
amorphous Al2O3 matrix without severe aggregation and
exhibit a uniform sphericalꢀlike shape. In addition, the
average sizes of CuꢀCo alloy nanoparticles range from 3.5
to 7 nm, to see the inset of particle size distribution in Fig.
5 (a), agrees with the average crystallite size calculated
from the XRD (listed in Tab. 1). The narrow particle size
distribution is attributed to the uniform distribution of
copper and cobalt ions in its precursor and to the effective
prevention of the Al2O3 matrix to the alloy particle
sintering.
Fig. 4 SEM images of (a) Cu2AlꢀLDHs, (b) (Cu1Co1)2AlꢀLDHs, (c)
Co2AlꢀLDHs, (d) (Cu1Co2)2AlꢀLDHs, (e) (Cu1Co2)2AlꢀCLDHs and
(f) (Cu1Co2)2AlꢀRed.
The morphology of the samples was investigated by
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accordance with that in the initial nitrate salt for preparing
the precursor.
Atomicꢀscale
chemical
composition
across
(CuxCoy)2AlꢀRed were investigated by HAADFꢀSTEMꢀ
EDS line scans, representative images and EDS curves are
shown in Fig. 5dꢀf and d’ꢀf’. The red lines in Fig. 5dꢀf in
the STEM pictures are the scanning area, which is
corresponding to the EDS curves in Fig. 5d’ꢀf’. The same
variation tendency for the two elements of copper and
cobalt can be seen clearly, meaning the formation of
homogeneous CuꢀCo alloy in all the three catalysts. Noting
that the intensity of the signal for copper and cobalt is
different in different samples, indicating the formed CuꢀCo
alloy with different Cu/Co ratio, which is in good
accordance with the XRD and TPR results.
Studies on the phase diagram of binary CoꢀCu suggest
that the maximum solubility of copper in metal cobalt is
9 % in molar ratio48. It should be noted that this solubility
is for the bulk alloy, that is, for the alloy with large
crystalline size. While for the nanosized bimetallic CuꢀCo,
16
the situation should be different. Prieto et al.
demonstrated that the superior nanoscale mixing of Cu and
Co species can promote the formation of CuꢀCo alloy
nanocrystals after activation, and pointed that the
Fig. 5 (a) TEM (insets: a HRTEM image and the particle size
distribution diagram), (b) EDX mapping, (c) EDX spectrum of the
(Cu1Co2)2AlꢀRed observed on a Mo substrate. Si comes from quartz
sand. STEM images and the corresponding EDS line scanning
profiles for Cu and Co elements of (d) and (d’) (Cu1Co1)2AlꢀRed, (e)
and (e’) (Cu1Co2)2AlꢀRed, (f) and (f’) (Cu1Co3)2AlꢀRed.
maximum occurrence of the CuꢀCo alloy phase is at a
18
Cu/Co molar ratio of 0.5. Volkova et al.
also found
singleꢀphase CuꢀCo alloy with the alloy composition
Cu0.5Co0.5 after lowꢀtemperature reduction of CuCoO2.
For further insight into the distribution of copper and
cobalt, Energy Dispersive Xꢀray Spectrometry Mapping
Analysis (Fig. 5(b)) was performed. Noting that, Cu and
Co show the same morphology of distribution, meaning
the uniform and homogeneous distribution of Cu and Co
elements throughout the Al2O3 matrix. The energy
dispersive Xꢀray spectrometry (EDX) analysis in Fig. 5 (c)
for (Cu1Co2)AlꢀRed reveals the coꢀexistence of Cu, Co and
Al elements, and the Cu/Co and (Cu+Co)/Al molar ratio
are 0.49 and 2.08, respectively, which is in good
3.6Catalytic performance
In the process of higher alcohols synthesis from syngas
over CuCoꢀbased catalysts, several parallel reactions
including FischerꢀTropsch synthesis and waterꢀgasꢀshift
reaction (WGSR) can be occurred. It is widely accepted
that HAS requires both dissociative and nonꢀdissociative
activation of CO on different active sites. Cu can act as the
active sites for CO molecular activation and insertion,
while Co for CO dissociative activation and chain
propagation. An appropriate balance between CO
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dissociation and CO insertion was suggested to be samples. The formation of alcohols and hydrocarbons are
necessary for the synthesis of higher alcohols. So the accompanied with the production of water, and water
synergetic effect of homogeneously dispersed copper and reacted with CO via WGSR generated CO2 50. With the
cobalt species plays a key role in the catalytic performance. temperature increase, the content of water increased
markedly with the increase of CO conversion, in turn
accelerating WGSR and CO2 generation.
The catalytic performance results present in Table 2.
The selectivity to higher alcohols increased from 42.3% of
Cu/Co = 1:3 to 44.1% of Cu/Co = 1:2, and then went down
to 42.6% of Cu/Co = 1:1. (Cu1Co2)2AlꢀRed exhibit the
highest selectivity toward higher alcohols, likely owing to
that in which most copper and cobalt were in the state of
alloy, to note that the ratio of Cu/Co in (Cu1Co2)2AlꢀRed
equals to that in CuCo2O4.
It is worth mentioning that the selectivity toward
Fig. 6 The conversions of CO and selectivity toward hydrocarbons,
higher alcohols is higher than that over previously reported
CuꢀCoꢀbased catalysts 14, 51, which should be due to the
(●) (Cu1Co2)2AlꢀRed and (▲) (Cu1Co3)2AlꢀRed at GHSV of 3900
CO2 and alcohols vs reaction temperature over (■) (Cu1Co1)2AlꢀRed,
mL (gcat h)ꢀ1 in the syngas mixture of H2/CO/N2 = 8/4/1 and at 3 MPa.
formation of CuꢀCo alloy and the uniform mixing of the
Fig. 6 shows the results of catalytic performance over
(CuxCoy)2AlꢀRed catalysts with different Cu/Co ratio. It is
shown that the conversion of CO and the selectivity
towards hydrocarbons increase with increasing cobalt
content and the reaction temperature over the three
catalysts. This agrees with the fact that metallic Co0 is the
active component for FꢀT reaction, and is responsible for
the dissociative CO adsorption, CꢀC chain growth and
hydrogenation. The selectivity of hydrocarbons increasing
with reaction temperature is ascribed to the high activation
energy for the dissociative adsorption of CO, which is
favorable for hydrocarbon generation at higher
elements in the catalyst. In the alloyed phase, the two
active sites are uniformly distributed and extremely close
to each other. This can significantly increase the
synergistic effect between copper and cobalt, which
contributed to the enhancement of the selectivity to higher
alcohols, as stated in the literatures 14, 16, 34
.
Over (Cu1Co2)2AlꢀRed and at 250ºC, 3MPa and
GHSV of 3900 mL(gcath)ꢀ1, CO conversion is 51.8%, the
selectivity to alcohols is 45.8%, and 94.3 wt. % of the
alcohols is C2+OH. The catalyst is very active and
especially exhibited much high selectivity to C2+OH.
The stability test results for (Cu1Co2)2AlꢀRed are
shown in Fig. 7. The CO conversion and the selectivity to
alcohols substantially remain stable in the 200 hours’
reaction process. After reaction for 200 h, the conversion
of CO is 53.1%, the selectivity to alcohols is 38.4%, and
the mass faction of higher alcohols in total alcohols is 92.8
wt.%.
temperatures 3, 15
.
With the increase of copper content, the level of CO2
production increase accordingly. This is in accordance with
the fact that Cu is more active than Co for WGSR 49. In
addition, an increased selectivity towards CO2 with
increasing reaction temperatures can be observed for all
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Table 2 Catalytic performance of CO hydrogenation over (CuxCoy)2AlꢀRed a.
ARTICLE
XCO
%
SRH
%
SCO2
%
ROH distribution
Catalyst
CH3OH
76.2
2.0
C2H5OH
2.1
C3H7OH
0.9
C4+OH
0.5
Cu2AlꢀRed
(Cu1Co1)2AlꢀRed
(Cu1Co2)2AlꢀRed
(Cu1Co3)2AlꢀRed
Co2AlꢀRed
35.7
11.8
50.1
51.3
53.5
93.4
8.5
5.3
2.9
2.4
1.2
43.2
51.8
54.0
63.5
18.0
18.3
18.5
0.8
15.6
16.9
14.6
1.4
9.0
1.7
8.9
1.8
9.2
0.5
2.7
a Reaction conditions: 250ºC, GHSV = 3900 mL (gcat h)ꢀ1, 3 MPa, H2/CO/N2= 8:4:1.
Table 3 CO conversions, alcohols selectivities and the corresponding reaction conditions for some representative catalysts reported in
references
Temperature
(°C)
GHSV
(hꢀ1)
7200 c
18000 c
2000
1800 c
1800.c
3900.c
3900.c
XCO
(%)
5.7
SROH
(%)
C2+OHb
%
Catalysts
H2/COa
Pressure (MPa)
CoꢀCu 49
Co@Cu 42
CuFeMg 28
240
1.5
2
4
2
4
2
2
3
3
37.9
33.0
49.1
23.3
38.2
45.8
38.4
37
230
─
91.4
77.0
79.3
88.1
94.3
92.8
300
2
56.89
23.2
47.6
51.8
53.1
14
d
CuꢀCo/Al2O3
CuꢀCo/Al2O3
CuꢀCo/Al2O3
CuꢀCo/Al2O3
250
2
250
2
e
250
2
f
260
2
a The molar ratio of H2/CO. b The mass fraction of C2+OH including ethanol in the total alcohols. c The unit is mL (gcat h)ꢀ1.dꢀe In this work.f
This work in stability test.
catalyst before reaction to about 6.2 nm for the catalyst
after the stability test.
The catalytic performance of some representative
excellent catalysts reported in literatures is listed in Table 3.
Taken activity and selectivity into consideration, CuꢀCo
alloy derived from (CuxCoy)2Al ꢀ LDHs turns out to be one
of the best catalysts for HAS. In particular, compared with
that of Ref 14, under the same reaction conditions, the asꢀ
Fig. 7 Conversion of CO (■) and selectivity to alcohols (▼), prepared catalysts in our work shows much better catalytic
hydrocarbons (●), CO2 (▲) over (Cu1Co2)2AlꢀRed with reaction
performance, whether the conversion of CO or the
time on stream at T = 260 ºC, P = 3 MPa, GHSV = 3900 mL (gcat h)ꢀ1
selectivity towards higher alcohols.
and H2/CO/N2 = 8/4/1. The inset picture is the XRD profiles of (a)
4.Conclusions
(Cu1Co2)2AlꢀRed and (b) (Cu1Co2)2AlꢀRed used for 200h, (♦) CuꢀCo
Layered double hydroxides containing copper, cobalt
and aluminum ions with well crystallized hydrotalcite
structure were prepared by using coꢀprecipitation method.
After calcination, the resultant oxides particles were
uniformly mixed in size of several nanometers and
CuCo2O4 is one of the main phases, owing to the
topological effect of LDHs. In the reduction process, the
alloy.
The inset picture in Fig. 7 is the XRD profiles of (a)
(Cu1Co2)2AlꢀRed and (b) (Cu1Co2)2AlꢀRed used for 200h.
By comparing the diffraction peaks of CuꢀCo alloy before
and after 200h reaction, it is seen that the nanoparticles of
CuꢀCo alloy show good antiꢀsintering ability. The
crystalline size only increased from about 4.8 nm the
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Financial support for this work was provided by the
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