Nanoparticles of Cu-Co alloy supported on high surface area LaFeO3 - Preparation and catalytic performance for higher alcohol synthesis from syngas

Article abstract of DOI:10.1039/c5ra02433a

LaFeO₃ supported Cu-Co bimetallic catalysts with high surface area and mesoporosity were prepared by impregnation combined with nanocasting method, and the resulting catalysts were used for higher alcohol synthesis (HAS) from syngas. These catalysts were characterized by using N₂ adsorption and desorption, X-ray diffraction, temperature programmed reduction, transmission electron microscopy and energy dispersive spectrometry techniques. The results showed that the catalysts were highly active and selective to higher alcohols, and meanwhile stable for HAS. Characterization results indicated that Cu-Co was in the state of a solid solution alloy in the reduced catalysts, and the formation of Cu-Co alloy led to the high selectivity to higher alcohols. The elements in the catalysts were uniformly mixed and in interaction, thus sintering of the nanoparticles in the catalysts was restricted, resulting in a good stability. The high activity was mainly attributed to the high surface area and mesoporosity. Compared with Cu-Co/LaFeO₃ prepared by conventional methods, Cu-Co/LaFeO₃ with higher surface area and mesoporosity exhibited better activity and higher selectivity to higher alcohols. This design scheme may be applied to other bimetallic catalysts, and many transition metal ions can act as the lattice ions of perovskite-type oxides besides copper and cobalt.

Full text of DOI:10.1039/c5ra02433a

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PAPER
Nanoparticles of Cu–Co alloy supported on high
surface area LaFeO3—preparation and catalytic
performance for higher alcohol synthesis from
syngas
Cite this: RSC Adv., 2015, 5, 31637
ab
Guilong Liu,ab Dongming Pan,c Ting Niu,ab Ang Cao,ab Yizhi Yueab and Yuan Liu
*
LaFeO3 supported Cu–Co bimetallic catalysts with high surface area and mesoporosity were prepared by
impregnation combined with nanocasting method, and the resulting catalysts were used for higher
alcohol synthesis (HAS) from syngas. These catalysts were characterized by using N2 adsorption and
desorption, X-ray diffraction, temperature programmed reduction, transmission electron microscopy and
energy dispersive spectrometry techniques. The results showed that the catalysts were highly active and
selective to higher alcohols, and meanwhile stable for HAS. Characterization results indicated that
Cu–Co was in the state of a solid solution alloy in the reduced catalysts, and the formation of Cu–Co
alloy led to the high selectivity to higher alcohols. The elements in the catalysts were uniformly mixed
and in interaction, thus sintering of the nanoparticles in the catalysts was restricted, resulting in a good
stability. The high activity was mainly attributed to the high surface area and mesoporosity. Compared
with Cu–Co/LaFeO3 prepared by conventional methods, Cu–Co/LaFeO3 with higher surface area and
mesoporosity exhibited better activity and higher selectivity to higher alcohols. This design scheme may
be applied to other bimetallic catalysts, and many transition metal ions can act as the lattice ions of
perovskite-type oxides besides copper and cobalt.
Received 7th February 2015
Accepted 26th March 2015
DOI: 10.1039/c5ra02433a
www.rsc.org/advances
Among the four types of catalysts, modied Fisher–
Tropsch catalysts are most attractive, owing to their high
1. Introduction
activity and comparatively high selectivity to higher alcohols
at milder reaction conditions.1,17 Modied Fisher–Tropsch
catalysts include Cu–Fe and Cu–Co based catalysts. For
Cu–Co based catalysts, it is accepted that metal copper can
activate CO non-dissociatively and subsequently generate
methanol, while metal cobalt enables the dissociative
adsorption of CO and C–C chain growth.18 Cu and Co work
cooperatively to generate higher alcohols, thus the interac-
tion between Cu and Co is critical on the catalytic perfor-
mance for the synthesis of higher alcohols.17,19,20 More
specically, it is recognized that the formation of Cu–Co alloy
is much important for the catalytic performance of Cu–Co
catalysts for HAS.19–21
To synthesize Cu–Co alloy, CuCoO2 (ref. 19) and CuCo2O4
(ref. 20 and 22) were used as the catalyst precursors, for that
in those compounds copper and cobalt species are uniformly
distributed at the atomic level which would lead to the
formation of alloy under reduction. Nevertheless, CuO and
Co3O4 could hardly be avoided during the formation of
CuCoO2 and CuCo2O4. Aer reduction, CuO and Co3O4 would
be converted to mono metallic copper and mono metallic
cobalt, the mono metals would generate methanol and
hydrocarbons, respectively.23
Higher alcohols, alcohols other than methanol, can be used as
fuels, fuel additives and intermediates for chemical feedstocks.1–3
Due to increasing oil prices and decreasing fossil resources,
higher alcohol synthesis from syngas or biomass has attracted a
considerable amount of attention.4 However, it is difficult to
commercialize this process due to the low selectivity to higher
alcohols or poor stability of the catalysts developed to date.5
Four types of catalysts were reported for HAS, namely, the
modied methanol catalysts, the modied Fisher–Tropsch cata-
lysts, the Mo-based catalysts and the Rh-based catalysts. Supported
Rh catalysts show relatively good activity and high selectivity to
ethanol;6–9 however the high price of Rh restricts their extensive
application. Alkali-modied Mo-based catalysts are attractive
owing to their excellent resistance to sulfur poisoning, however
they must be utilized at a strict ratio of H2/CO ¼ 1 : 1, high
temperature and pressure.10–12 As for the modied methanol
catalysts, methanol is the major product.13–16
aCollaborative Innovation Center of Chemical Science and Engineering, Tianjin
300072, China. E-mail: yuanliu@tju.edu.cn; Tel: +86 022 87401675
bTianjin Key Laboratory of Applied Catalysis Science and Technology, School of
Chemical Engineering, Tianjin University, Tianjin 300072, China
cNingbo Branch of China Academy of Ordnance Science, Baotou 014034, China
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Perovskite-type oxides (PTO) can be expressed as ABO3, catalysts, studies on PTOs as catalyst supports are few. Based on
where the metal ions at A and B sites can be substituted by other the special characters of PTO, bimetal nanoparticles can be
transitional metal ions. In LaCo1ꢀxCuxO3, copper and cobalt supported on metal oxides, which should be a promising and
ions are located in the lattice of perovskite structure and can be extensively used method for preparing supported
uniformly dispersed at the atomic level, thus it can also be bimetal catalysts.
converted to bimetallic Cu–Co nanoparticles supported on
Hence, a PTO can be used as the support for loading bime-
La2O3 aer reduction pretreatment.21,24,25 However, due to the tallic nanoparticles, such as, using LaFeO3 to load bimetallic
limited surface area of La2O3, Cu–Co nanoparticles tend to Cu–Co nanoparticles. As a catalyst support, large specic
sinter during reaction.25 Thus improvement on the stability of surface area is required. However, when preparing PTOs, high-
the catalyst is needed.
temperature calcination is a necessary step, resultantly the
To overcome the problem of the sintering of Cu–Co nano- surface area of PTO is generally low.
particles, in our previous works LaCo1ꢀxCuxO3 were loaded on
Hard template method can be used for preparing perovskite
SiO2 (ref. 26) and ZrO2 (ref. 27), both of the two supported with mesoporous structure and high surface area.28–31 Nair
catalysts showed good activity and high selectivity to higher et al.32,33 has reported the preparation of high surface area
alcohols. While, there are still some disadvantages. Firstly, LaMnO3, LaFeO3, LaCoO3 and LaNiO3 by using this method,
loading enough LaCo1ꢀxCuxO3 on a support is a hard work; and found that those catalysts showed better catalytic perfor-
secondly, since x in LaCo1ꢀxCuxO3 cannot exceed 0.3, adjusting mance than the corresponding perovskite with no meso-
the ratio of Cu–Co is limited; thirdly, the interaction between porosity. Wang et al.29 found mesoporous LaFe1ꢀxCoxO3 were
the metal and the support is not strong enough, so sintering of electrocatalytically active and highly stable for oxygen reduction
the metal nanoparticles is still a problem although it was reaction, owing to the Fe3+/Fe2+ redox couple and electrical
improved at a certain extent.
conductivity. As for the HAS catalyst, no report can be found
PTO can also be acted as the support for nanoparticles. For about the mesoporous perovskite.
example, replacing iron ions partly with nickel ions in LaFeO3
In this work, mesoporous LaFeO3 with high surface area was
results in LaFe1ꢀxNixO3; and furthermore, replacing part of iron used as the support for nano Cu–Co alloys and the catalytic
ions with cobalt ions in LaFe1ꢀxNixO3 will generate LaFe1ꢀxꢀy
-
performance of the supported catalysts for HAS were investi-
CoyNixO3. Under reduction, LaFe1ꢀxꢀyCoyNixO3 will be trans- gated. The results indicated that nanoparticles of Cu–Co alloys
formed to Ni–Co/LaFeO3, for that LaFeO3 is stable at the were made and the catalysts were very active, highly selective
reductive atmosphere and that nickel and cobalt ions in the and very stable for HAS.
PTO are also uniformly dispersed at atomic level. Meanwhile,
nickel and cobalt are derived from the lattice ions of the PTO,
the interaction between Ni–Co and the support is favorable,
helping the improvement for the anti-sintering ability of Ni–Co
2. Experimental
2.1. Catalyst preparation and materials
nanoparticles.
To take Cu–Co/LaFeO3 as another example, which is the Cu(NO3)2$3H2O, Fe(NO3)3$9H2O, ethanol, citric acid, tetraethyl
system studied in this work. Co3O4/LaFe1ꢀxCuxO3 can be used orthosilicate, butanol, and P123 ((C3H6O$C2H4O)x, EO20PO70
2.1.1. Material.
La(NO3)3$6H2O,
Co(NO3)2$6H2O,
-
as the precursor. Firstly, Co3O4 can be highly dispersed on the EO20, Ma ¼ 5800) were purchased from Aladdin Industrial
surface of LaFe1ꢀxCuxO3 grains, for that cobalt ions can be acted Corporation and used without further purication.
as the lattice ions of the PTO as stated in the above paragraph
2.1.2. Catalyst preparation. KIT-6 (a mesoporous silica
and Co3O4 tends to interact with LaFe1ꢀxCuxO3. Secondly, zeolite) was prepared according to the method in the literature34
copper ions in LaFe1ꢀxCuxO3 grains are uniformly dispersed. and used as the hard-template for preparing mesoporous-
Thus, during reduction, copper ions would be reduced to LaFe0.7Cu0.3O3. A homogeneous clear solution with a molar
metallic copper and migrate to the surface of LaFeO3 (with the composition of tetraethyl orthosilicate/P123/HCl/H2O/butanol
reduction of copper ions, LaFe1ꢀxCuxO3 would transform to ¼ 1/0.017/1.83/195/1.31 was stirring at 38 ꢁC for 24 h, and
LaFeO3), and metallic copper should be highly dispersed. At the then aged at 80 ꢁC for 48 hꢁ. The resulted products were ltered,
same time, the highly dispersed Co3O4 would be reduced to dried and calcined at 700 C, KIT-6 was obtained.
metallic cobalt. Both of the metallic copper and cobalt are
Mesoporous-LaFe0.7Cu0.3O3, labeled as LaFe0.7Cu0.3O3 (M),
highly dispersed on the surface of LaFeO3, the two kinds of was prepared by a simple co-nanocasting method with the
metals would be inclined to interact to form alloy nanoparticles. similar procedure reported by Wang et al.29 Lanthanum, copper
Thirdly, using Co3O4/LaFe1ꢀxCuxO3 as the precursor, the ratio and ferric nitrates with a molar ratio of 1 : 0.3 : 0.7 were dis-
of Co–Cu can be conveniently controlled by adjusting the solved in an ethanolic solution. Then citric acid was added into
loading amount of cobalt.
the above solution, and the molars of citric acid were equal to
It is known that about 90% of transitional metal ions can be that of the total nitrates. KIT-6 was added into the above solu-
acted as A site or B site ions of PTOs, which suggests that a lot of tion, and the solution was evaporated at 40 ꢁC. The obtained gel
alloy particles of bimetal or multi-metal can be supported on was dried, and then calcined at 700 ꢁC for 6 h. KIT-6 was
PTOs. Therefore, PTOs supported metal alloys possibly become removed by treating in a 2 M NaOH solution for three times.
a kind of valuable new catalysts. While studies on PTOs in LaFe0.7Cu0.3O3 (M) was obtained by centrifugation, washing and
catalysis area mainly are focused on PTOs themselves as drying the template-free samples. For comparison,
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LaFe0.7Cu0.3O3 was prepared according to the citrate complex- temperature, the reaction was maintained for 3 h. The gas
ing method which is the same as stated above, except that KIT-6 chromatograph was equipped with two packed columns. H2,
template was not used.
CO, CO2, N2 and CH4 of the off-gases were analyzed by using a
To load Co3O4 onto the support, incipient impregnation TDX-01 packed column (2 m) connected to a TCD detector. The
method was employed. Cobalt nitrate was dissolved in deion- hydrocarbons and condensed liquid products of alcohols were
ized water, then the support was soaked into thꢁe solution for 24 analyzed using a Porapak Q column (3 m) connected to a FID
h under a vacuum condition and dried at 80 C for 24 h in a detector.
ꢁ
vacuum oven. Finally, the dryꢁsamples were calcined at 350 C
CO conversion (XCO) and the product selectivity (Si) were
for 3 h at a heating rate of 2 C minꢀ1. The prepared catalysts calculated according to the following equations: XCO ¼ (MCOin
ꢀ
P
were labeled as x% Co3O4/LaFe0.7Cu0.3O3 or x% Co3O4/LaFe0.7
-
MCOout)/MCOin and Si ¼ nCi/ nCi, where MCOin, MCOout is the
Cu0.3O3 (M), where x% was the mass fraction of Co3O4 in the moles of CO in the feed-gases and off-gases, respectively; n, Ci
calcined catalysts. In the prepared catalysts, the Co–Cu molar represents the number of carbon atoms and moles of a carbon-
ratios were 0.19, 0.59 and 1.0 corresponding to x ¼ 2, 6 and 10, containing product, respectively.
respectively.
Before reaction,ꢁthe samples were heated to 310 ꢁC in N2 at a
3. Results
3.1. Specic surface area and pore size distribution
heating rate of 10 C minꢀ1, then switched to H2 atmosphere
and maintained in H2 at 310 ꢁC for 3 h.
The N2 adsorption–desorption isotherms and the correspond-
ing pore size distribution (the inset picture) of the catalysts are
shown in Fig. 1. The onset of the relative pressure of the
hysteresis loops for 6% Co3O4/LaFe0.7Cu0.3O3 is at a high posi-
tion, suggesting the macropores with the size over 50 nm
dominating in the sample. As for the other samples, the onset of
the relative pressure is close to 0.6, suggesting the existence of
mesopores. All of the pores in the samples are formed by
particles packing, and not in parallel arranged structure.
The surface areas and the most frequency pore sizes were
calculated from the N2 adsorption and desorption isotherms,
and the results are listed in Table 1. The surface areas and the
pore sizes of KIT-6 and LaFe0.7Cu0.3O3 (M) are on the same level
as compared with those reported in the literature.29 Seen from
Table 1, aer introducing of mesoporosity into LaFe0.7Cu0.3O3,
the surface area of x% Co3O4/LaFe0.7Cu0.3O3 (M) is greatly
improved than 6% Co3O4/LaFe0.7Cu0.3O3.
The pore size decreases with the increase of the loading
amount of Co3O4, indicating that subsequently impregnated
Co3O4 entered the space between perovskite particles, or in
other words, the Co3O4 was coated and highly dispersed on the
surface of LaFe0.7Cu0.3O3 (M) nanoparticles. In this way, Co3O4
would contact with LaFe0.7Cu0.3O3 (M) closely and the interac-
tion between them should be favored. Aer reduction, Cu–Co
2.2. Catalyst characterization
X-Ray diffraction (XRD) measurements were performed on a
Bruker D8-Focus X-ray diffractometer with Ni-ltered Cu Ka
radiation (l ¼ 0.15406 nm). The spectra were collected between
10ꢁ and 90ꢁ (2q degree) at a scanning speed of 5ꢁ minꢀ1
.
Nitrogen adsorption and desorption isotherms were per-
formed on a Trwastar 3000 micromeritics apparatus at ꢀ196 ꢁC.
The specic surface areas were calculated based on the BET
method and the pore size distributions were derived from the
adsorption branch of the isotherms using the BJH model. All
samples were outgassed under vacuum at 300 ꢁC for 4 h prior to
analysis.
Transmission electron microscopy (TEM) images, and high
angle annular dark eld scanning transmission electron
microscopy (HAADF-STEM) line scans were obtained on a JEOL
JEM-2100F microscope eld-emission transmission electron
microscope. Aer ultrasonic dispersion of the catalysts in
ethanol, the samples were deposited onto Ni grids covered by
holey carbon lm.
Temperature programmed reduction (TPR) experiments
were carried out in a quartz tube reactor. In each run, 50 mg
catalyst was loaded into a quartz tube reactor, purged for 30 min
with 5% H2/Ar to remove the air in the reactor, and then heated
to 7ꢀ001 ꢁC from room temperature at a heating rate of 10 ꢁC
min in the presence of 5% H2/Ar at a ow rate of 30 mL
minꢀ1. TCD was used as the detector.
2.3. Catalytic performance test for CO hydrogenation
Catalytic performance was evaluated in a stainless-steel xed-
bed continuous-ow reactor and gas chromatograph (GC)
combination system. In each run, 800 mg catalyst was loaded
into the reactor and then reduced at 310 ꢁC for 3 h in the
presence of H2 at a ow rate of 30 mL minꢀ1. Aer reduction,
the reactor was cooled to room temperature, and the pressure
was increased to 3 MPa by feeding the syngas mixture of H2/CO/
N2 ¼ 8 : 4 : 1 in volume, where N2 was used as the internal
Fig. 1 N2 adsorption–desorption isotherm (a) and BJH pore size
distribution (b) of (1) 2% Co3O4/LaFe0.7Cu0.3O3 (M), (2) 6% Co3O4/
LaFe0.7Cu0.3O3 (M), (3) 10% Co3O4/LaFe0.7Cu0.3O3 (M), (4) 6% Co3O4/
LaFe0.7Cu0.3O3 (M) after 200 hours' stability test and (5) 6% Co3O4/
standard gas for analyzing the reacted gases. The gas hourly
space velocity was set to 3900 mL gcat hꢀ1. At each reaction LaFe0.7Cu0.3O3.
ꢀ1
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Table 1 Physical properties of the support and catalysts
Surface area (m2 gꢀ1
)
Pore size (nm)
Crystal size of LaFe0.7Cu0.3O3 (nm)
a
Samples
KIT-6
820
6.5
—
LaFe0.7Cu0.3O3 (M)
133
9.6
7.6
2% Co3O4/LaFe0.7Cu0.3O3 (M)
6% Co3O4/LaFe0.7Cu0.3O3 (M)
6% Co3O4/LaFe0.7Cu0.3O3
10% Co3O4/LaFe0.7Cu0.3O3 (M)
131
9.4
7.5
128 (106b)
30
8.6 (7.5b)
—
8.4 (8.5b)
13.0
8.0
119
7.6
a
Crystal sizes were calculated by X-ray diffraction with Scherrer equation. b Data of 6% Co3O4/LaFe0.7Cu0.3O3 (M) aer 200 hours' stability test.
nanoparticles should be contacted with LaFeO3 and separated cobalt are the characteristics of the formation of Co–Cu alloys.
by the LaFeO3 particles, thus the sinter of Cu–Co nanoparticles Thus the weak diffraction peaks observed at around 43.5ꢁ in
would be restrained.
Fig. 2b can be attributed to Cu–Co alloy. Diffraction peaks for
Meanwhile, as the loading amount changes, only much little cobalt species cannot be detected, which may also imply the
variation of the specic surface area can be found. It can be formation of Cu–Co alloy, as stated by Subramanian et al.37
ascribed to the small loading amount of Co3O4 and the high Since the ratio of La/(Fe + Cu) ¼ 1 : 1, therefore accompanying
dispersion of Co3O4 on the surface of the PTO.
3.2. XRD
Fig. 2a illustrates the XRD patterns of LaFe0.7Cu0.3O3 (M) and
the supported catalysts with various Co3O4 content aer calci-
nation. Clear diffraction peaks of perovskite can be seen in all
samples. Compared with the diffraction peaks of LaFeO3, all the
diffraction peaks of the copper containing perovskite shi to
lower 2q values. The size of Cu2+ is bigger than the size of Fe3+
,
when Fe3+ was partially replaced by Cu2+, the crystal cells of the
perovskite became bigger, resulting in the shi of 2q to the
lower values.35 Conversely, the shi of the diffraction peaks
indicates that the copper ions have entered into the perovskite
lattice. The XRD results show that LaFe0.7Cu0.3O3 with perov-
skite structure was made.
The diffraction peaks corresponding to cobalt oxide can
hardly be seen for the supported catalysts, meaning that Co3O4
on the surface of the perovskite can achieve a high dispersion.
Meanwhile, to the best of our knowledge,29 cobalt ions in cobalt
oxide can enter into the crystal lattice of perovskite only when
the calcined temperature is higher than 500 ꢁC. In this work, the
catalysts of Co3O4/LaFe0.7Cu0.3O3 were calcined at 350 ꢁC, so
cobalt ions should not enter the crystal lattice of the PTO. That
is, the calcined catalysts are in the composition of Co3O4/
LaFe0.7Cu0.3O3 with Co3O4 being highly dispersed.
The samples of 6% Co3O4/LaFe0.7Cu0.3O3 and x% Co3O4/
LaFe0.7Cu0.3O3 (M) were reduced at 310 ꢁC for 3 h in 5% H2/Ar,
and the XRD proles of the reduced samples are demonstrated
in Fig. 2b. Aer reduction, the diffraction peaks of perovskite
still can be clearly seen, and the diffraction peaks shi to higher
2q values, because of the smaller lattice spacing of LaFeO3 than
LaFe0.7Cu0.3O3. Similarly, the shi of 2q values indicates that
the copper ions in LaFe0.7Cu0.3O3 were reduced and separated Fig. 2 XRD patterns of (1) LaFe0.7Cu0.3O3 (M), (2) 2% Co3O4/LaFe0.7
-
-
Cu0.3O3 (M), (3) 6% Co3O4/LaFe0.7Cu0.3O3 (M), (4) 6% Co3O4/LaFe0.7
out from the PTO lattice.
Cu0.3O3, (5) 10% Co3O4/LaFe0.7Cu0.3O3 (M) after calcination (a), after
reduction (b), after reaction (c) and 6% Co3O4/LaFe0.7Cu0.3O3 (M) after
200 hours' stability test (6) in (c). ()) LaFe0.7Cu0.3O3; (&) LaFeO3; (()
Co3O4; (O) Cu–Co alloy; (P) La2O3; (C) Cu; (,) Co2C; (A)
Su et al.2 attributed the diffraction peaks between Co (111) at
44.2ꢁ and Cu (111) at 43.3ꢁ to the bimetallic Co–Cu alloy.
Nguyen et al.36 demonstrated that the slight increase in lattice
spacing for copper and the small decrease in lattice spacing for La2O2CO3.
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the reduction of copper ions, there would be some La2O3 the TPR results are close to the theoretic H2 consumptions of
generated from LaFe0.7Cu0.3O3, and the diffraction peaks of Co3+ / Co2+ and Fe4+ / Fe3+. This supports the attributions of
La2O3 can also be observed in the XRD proles in Fig. 2b. Thus, the TPR peaks above.
aer reduction, 6% Co3O4/LaFe0.7Cu0.3O3 and x% Co3O4/
LaFe0.7Cu0.3O3 (M) were changed to Cu–Co/La2O3–LaFeO3 with moves gradually to a high temperature, meaning the higher
Cu–Co being in alloy state.
reduction temperature of Cu2+ / Cu0 and Co2+ / Co0. Tien-
With the increase of the loading amount of Co3O4, b peak
For the catalysts aer reaction, the diffraction peaks of Thao21 attributed the increase of the reduction temperature of
La2O2CO3 and Co2C can be seen in Fig. 2c. The clearly shown Cu2+ to the mutual interaction between Con+ and Cu2+. So, the
diffraction peaks of LaFeO3 indicate that LaFeO3 was stable in higher reduction temperature of Cu2+ to Cu0 may be an indic-
the reaction process. La2O2CO3 came from La2O3 which reacted ative of the interaction between Cu2+ and Con+. The interaction
with CO2 in the reaction conditions, while Co2C was derived between copper and cobalt favors the formation of Cu–Co alloy.
from the reaction between Co and C. Among the phases, Cu14,38
The reduction proles of 6% Co3O4/LaFe0.7Cu0.3O3 and 6%
and Co2C19 were regarded as the main active phase for alcohols Co3O4/LaFe0.7Cu0.3O3 (M) are much similar, suggesting that
synthesis. Simultaneously, La2O2CO3 can eliminate the carbon introducing mesoporosity into LaFe0.7Cu0.3O3 does not change
deposited, which is helpful for improving the stability of the the reduction behavior of the catalysts loaded with Co3O4.
catalyst.
From the TPR results it is seen thatꢁthe temperatures for all
of the reduction peaks are below 310 C, see Fig. 3. Thus, the
temperature for the reduction treatments was set at 310 ꢁC, and
aer the reduction the catalysts could be converted to Cu–Co/
La2O3–LaFeO3.
3.3. TPR
TPR results of the samples are demonstrated in Fig. 3. Two
main peaks (a and b) can be observed from 200 ꢁC to 350 ꢁC for
all the samples. In LaFe1ꢀxCuxO3 some iron ions changed from
Fe3+ to Fe4+, and in order to maintain electrical neutrality there
are some oxygen vacancies, as pointed out in the literature.39
The Fe4+ can be reduced at lower temperature around
3.4. TEM
As can be seen clearly in Fig. 4a, all the particles are uniformly
distributed and particle sizes are almost the same. The particle
size of LaFe0.7Cu0.3O3 (M) is around 10 nm, which agrees with
the XRD results above.
200 ꢁC,39,40 while Cu2+ in LaFe1ꢀxCuxO3 can be reduced at
around 300 C.35 Thus, the main reduction peak (b) in LaFe0.7
-
ꢁ
Cu0.3O3 and LaFe0.7Cu0.3O3 (M) at 280 ꢁC can be contributed to
the redꢁuction of Cu2+ / Cu0, while the small reduction peak (a)
at 250 C is corresponding to the reduction of Fe4+ to Fe3+.
It is generally accepted that Co3O4 is reduced by two steps,
the rst at a lower temperature is corresponding to Co3+ / Co2+
and a higher temperature is attributed to Co2+ to metal
cobalt.41,42 Therefore, for the catalysts loading Co3O4, the rst
˚
Lattice distance of 2.78 A can be observed in Fig. 4b–e, which
˚
is the lattice parameter of LaFeO3 [121] planes (2.778 A).
Namely, aer reduction, LaFeO3 was formed in all samples. The
˚
lattice distance of 1.78 A is contributed to [200] planes of the
Cu–Co alloy for the reduced samples (Fig. 4b–e), meaning that
aer reduction Cu–Co alloy can be obtained in all samples.
˚
reduction peak (a) can be attributed to the reduction of Co3+
/
Meanwhile, [111] planes for Cu–Co alloy (2.07 A) can also be
Co2+ and Fe4+ to Fe3+; while the second reduction peak (b) can
be owned to the reduction of Co2+ / Co0 and Cu2+ / Cu0.
H2 consumptions were calculated and listed in Table 2. The
H2 consumptions of b peaks derived from the TPR results are
close to the total theoretic H2 consumptions of Co2+ / Co0 and
Cu2+ / Cu0, and the H2 consumptions of a peaks derived from
observed in Fig. 4c.
˚
˚
In Fig. 4c and d, the lattice distances of 1.975 A and 2.28 A
are in accordance with the lattice parameters for [100] and [102]
planes of La2O3, respectively. Moreover, it is seen that La2O3 was
mixed with Cu–Co alloy uniformly.
At this aspect, Co3O4/LaFe0.7Cu0.3O3 and Co3O4/LaFe0.7
-
Cu0.3O3 (M) with mesoporosity are the same, for both of them,
aer reduction Cu–Co alloy/La2O3–LaFeO3 were obtained.
These results are in agreeing with XRD and TPR results above.
As can be seen in Fig. 4f, for 6% Co3O4/LaFe0.7Cu0.3O3 (M)
aer reduction, particle agglomeration can hardly be observed.
The particles are well dispersed and the sizes are about 10 nm.
Most importantly, aer 200 hours' stability test, no obvious
particle agglomeration can be found in 6% Co3O4/LaFe0.7
-
Cu0.3O3 (M) (Fig. 4h), meaning that the catalysts prepared by
this method is stable enough under the reaction conditions of
HAS. However, aer reduction of 6% Co3O4/LaFe0.7Cu0.3O3, the
particle size is much large (Fig. 4g), and the crystalline sizes of
LaFeO3 and Cu–Co alloy are 11.9 and 9.6 nm, respectively,
which are a little larger than that of 6% Co3O4/LaFe0.7Cu0.3O3
(M) with mesoporosity, indicating severe agglomeration. This
agglomeration was not ascribed to the reduction, for that the
Fig. 3 TPR results of (1) LaFe0.7Cu0.3O3, (2) LaFe0.7Cu0.3O3 (M), (3) 2%
Co3O4/LaFe0.7Cu0.3O3 (M), (4) 6% Co3O4/LaFe0.7Cu0.3O3 (M), (5) 6%
Co3O4/LaFe0.7Cu0.3O3 and (6) 10% Co3O4/LaFe0.7Cu0.3O3 (M).
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Table 2 Hydrogen consumptions for the catalysts in TPR process
H2 consumptions from
TPR resultsa,b
Theoretic H2 consumptionsb
Samples
a
b
Fe4+ / Fe3+
Cu2+ / Cu0
Co3+ / Co0
10% Co3O4/LaFe0.7Cu0.3O3 (M)
6% Co3O4/LaFe0.7Cu0.3O3
6% Co3O4/LaFe0.7Cu0.3O3 (M)
2% Co3O4/LaFe0.7Cu0.3O3 (M)
LaFe0.7Cu0.3O3 (M)
0.046
0.039
0.040
0.027
0.005
0.016
0.110
0.086
0.081
0.067
0.054
0.050
0.028
0.029
0.029
0.030
0.030
0.030
0.056
0.058
0.058
0.060
0.061
0.061
0.076
0.047
0.047
0.017
0
LaFe0.7Cu0.3O3
0
a
Experimental H2 consumptions calculated from TPR results using CuO as the reference material. b The unit is mmol H2 per 50 mg.
Fig. 5. The red lines in the STEM pictures are the scanning
area, which is related to the EDS curves. Abscissas in EDS
curves represent the scanning distance from one end to the
other side of the particles, and the signals were varying in
intensity with the regions of the nanoparticles can be seen
clearly.
The same variation tendency of the two elements of copper
and cobalt can be seen in Fig. 5a clearly, meaning the uniform
mixing of copper and cobalt in this sample. Namely, Cu and Co
were in forms of homogeneous alloy or uniform mixture.
However, no diffraction peaks of Cu and Co but a diffraction
peak between Cu and Co could found in Fig. 2b; moreover, TEM
results indicate the formation of Cu–Co alloy and TPR results
support the Cu–Co alloy formation. Thus, Cu and Co are in
forms of alloy rather than uniform mixture.
Aer 200 hours' stability test, uniform mixing of copper and
cobalt can still be seen in Fig. 5b. The appearance of copper is
accompanied by the appearance of cobalt, and vice versa.
Although, aer reaction, Cu–Co alloy was changed to metal
copper and Co2C as the XRD results indicated, the uniform
mixture of copper and cobalt maintained.
3.5. Catalytic performance
The catalytic performance of the prepared catalysts for HAS
from syngas are shown in Fig. 6 and Table 3. The conversion
of CO and the formation of hydrocarbons (CxHy) increase
with the increase of temperature, which is attributed to the
high activation energy for the dissociative adsorption of CO,
and the dissociative adsorption of CO is favorable for
hydrocarbon generation at higher temperatures.26 The
Fig. 4 TEM pictures of 6% Co3O4/LaFe0.7Cu0.3O3 (M) (a) and 2%
Co3O4/LaFe0.7Cu0.3O3 (M) (b), 6% Co3O4/LaFe0.7Cu0.3O3 (M) (c and f),
selectivity to CO2 increased with the increase of temperature,
because of that H2O would be produced during HAS, and
water could accelerate water gas shi reaction which produce
CO2.36 The selectivities to alcohols decrease with the increase
of temperature, for that the rate-determining-step for alco-
hols synthesis is the formation of acyl species on Cu sites,
which is an exothermic reaction.4,12 The conversion of CO
increased with the increase of the amount of Co3O4 loaded,
for that cobalt is the active component for dissociative
adsorption of CO.43,44
10% Co3O4/LaFe0.7Cu0.3O3 (M) (d), 6% Co3O4/LaFe0.7Cu0.3O3 (e and g)
after reduction and 6% Co3O4/LaFe0.7Cu0.3O3 (M) after stability test (h).
particles in the calcined 6% Co3O4/LaFe0.7Cu0.3O3 was severely
agglomerated.
Atomic-scale chemical composition across 6% Co3O4/
LaFe0.7Cu0.3O3 (M) aer reduction and aer 200 hours'
stability test were investigated by HAADF-STEM-EDS line
scans, representative images and EDS curves are shown in
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Fig. 5 STEM images and the corresponding EDS line scanning profiles for Cu and Co elements of 6% Co3O4/LaFe0.7Cu0.3O3 (M) after reduction
(a and a') and after 200 hours' stability test (b and b').
Compared to 6% Co3O4/LaFe0.7Cu0.3O3 (M), over 6% Co3O4/ specic surface area of 6% Co3O4/LaFe0.7Cu0.3O3. At the high
LaFe0.7Cu0.3O3 CO conversions at low temperatures are obvi- temperature of 340 and 350 ꢁC, CO conversions over the two
ously lower, which is due to the agglomeration and the lower catalysts are close, both reached about 100%. At such a high CO
Fig. 6 The variation of CO conversions and selectivity to CO2, hydrocarbons and alcohols with reaction temperature over ( ) 2% Co3O4/
LaFe0.7Cu0.3O3 (M); ( ) 6% Co3O4/LaFe0.7Cu0.3O3 (M); ( ) 6% Co3O4/LaFe0.7Cu0.3O3 and ( ) 10% Co3O4/LaFe0.7Cu0.3O3 (M) at GHSV of
3900 mL gcatꢀ1 hꢀ1 in the syngas mixture of H2/CO/N2 ¼ 8/4/1 and at pressure of 3 MPa.
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Table 3 Catalytic performance of CO hydrogenation over Cu–Co/La2O3–LaFeO3 catalystsa
Selectivity to HC/%
Selectivity to alcohols/%
Samples
XCO (%)
Sco (%)
C1
C2
C3
C4+
C1
C2
C3
C4+
2
2% Co3O4/LaFe0.7Cu0.3O3 (M)
6% Co3O4/LaFe0.7Cu0.3O3 (M)
6% Co3O4/LaFe0.7Cu0.3O3
39.9
50.9
27.7
76.1
13.8
18.8
10.9
25.9
11.5
8.4
18.2
7.0
22.5
26.9
26.2
24.6
5.5
6.6
6.4
7.0
0
13.8
8.1
13.7
6.4
26.6
25.1
19.5
22.7
6.3
6.0
4.8
5.9
0
0
0.1
0.1
0.1
0.2
0.4
10% Co3O4/LaFe0.7Cu0.3O3 (M)
a
ꢀ1
Reaction conditions: 300 ꢁC, GHSV ¼ 3900 mL gcat hꢀ1, 3 MPa, H2/CO/N2 ¼ 8 : 4 : 1.
conversion, the catalysts are active enough and the specic
surface area has a small effect on CO conversion.
Some representative excellent catalysts reported in litera-
tures have been shown in Table 4. Even though the activity and
the selectivity of the catalysts in this work are not the best, taken
Higher selectivity to higher alcohols on 2% Co3O4/LaFe0.7
-
Cu0.3O3 (M) can be observed, however the conversion of CO is activity and selectivity into consideration, Cu–Co/La2O3–LaFeO3
rather low. Oppositely, over 10% Co3O4/LaFe0.7Cu0.3O3 (M), reduced from 6% Co3O4/LaFe0.7Cu0.3O3 (M) turns into one of
selectivity to higher alcohols is relatively lower and CO conver- the best catalysts for HAS.
sion is relatively high. Taken activity and selectivity into
consideration, 6% Co3O4/LaFe0.7Cu0.3O3 (M) becomes the best
catalyst among the three catalysts for higher alcohols synthesis.
4. Discussions
Compared with 6% Co3O4/LaFe0.7Cu0.3O3, over 6% Co3O4/
LaFe0.7Cu0.3O3 (M) besides obvious higher CO conversion is
observed, the selectivity to higher alcohols is a little higher and
the selectivity to hydrocarbons is lower while the selectivity to
CO2 is higher. The key difference between the two catalysts is
the porous structure, which has an effect on CO conversion and
selectivities. However, the detail mechanism of the effect is
difficult to explain, because of that HAS is a complex network
including many reactions, such as methanation, methanol
synthesis, carbon chain growth, water gas shi reaction, etc.
The stability test results for 6% Co3O4/LaFe0.7Cu0.3O3 (M) are
shown in Fig. 7. During reaction, the structure of bimetallic
Co–Cu may be varying, resultantly, the CO conversion and the
selectivities were varying. The CO conversion and the selectivity
to alcohols decreased a little in the beginning 50 hours. And
then the CO conversion and the selectivity to alcohols
substantially remained stable or even increased in the following
150 hours. Aer reaction for 200 hours, the conversion of CO is
47.1%, the selectivity to alcohols is 37%, and the mass faction of
higher alcohols in total alcohols is 79.5%.
In this paper, a series of Cu–Co/LaFeO3 were made and used for
higher alcohols synthesis. CuO/LaFe1ꢀxCoxO3, Co3O4/LaFe1ꢀy
CuyO3 or LaFe1ꢀxꢀyCoxCuyO3 can be used as the precursor for
-
preparing Cu–Co/LaFeO3. However when cobalt ions was placed
into the perovskite lattice, whether in forms of CuO/LaFe1ꢀx
-
CoxO3 or LaFe1ꢀxꢀyCoxCuyO3, the reduction of the cobalt ions
ꢁ
would only be occurred at least 400 C, the reduction behavior
agrees with that reported in the literatures.25,45 Thus, Cu2+ and
Con+ could not be reduced together easily and the formation of
Cu–Co alloy is not favored. When cobalt ions is in Co3O4, which
can be reduced to metallic Co at about 300 ꢁC as shown in Fig. 3,
and this agrees with the reports in the literatures.2,46 As a result,
both of copper ions and cobalt ions can be reduced to metallic
ꢁ
copper and metallic cobalt at around 310 C (Fig. 3) in Co3O4/
LaFe1ꢀyCuyO3. Hence, the formation of Cu–Co alloy can be
realized. For this reason, Co3O4/LaFe0.7Cu0.3O3 was used as the
precursor for Cu–Co/LaFeO3.
In order to form Cu–Co alloy, optimization on reduction
conditions was also proposed. As stated in Section 2.1, during
ꢁ
the reduction, all the samples was heated to 310 C in N2 at a
heating rate of 10 C minꢀ1, then switched to H2 atmosphere
ꢁ
2+
ꢁ
and maintained in H2 at 310 C for 3 h. In this way, Cu and
Con+ can be reduced into metallic form simultaneously.
4.1. Structure of the catalysts
From the characterization results including surface area,
structure of porosity, XRD, TPR and TEM, the following valuable
information can be achieved.
(1) From the XRD and TEM results, it is seen that Cu–Co
alloy supported on La2O3-doped LaFeO3 can be prepared by
using Co3O4/LaFe0.7Cu0.3O3 as the precursor, owing to the
uniform mixing of copper and cobalt ions in the precursor.
What is more interesting is that uniform mixing of copper
and cobalt species maintains even aer 200 hours' reaction
for HAS (Fig. 5b0), although during reaction Cu–Co alloys
Fig. 7 Conversion of CO ( ) and selectivity to alcohols ( ), hydro-
carbons ( ), CO2 ( ) over 6% Co3O4/LaFe0.7Cu0.3O3 (M) with reaction
time on stream at T ¼ 300 ꢁC, P ¼ 3 MPa, GHSV ¼ 3900 mL gcatꢀ1 hꢀ1
and H2/CO/N2 ¼ 8/4/1.
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Table 4 CO conversions and alcohols selectivities in some representative catalysts reported in references
Catalyst
Temperature (ꢁC)
H2/COa
Pressure (MPa)
GHSV (hꢀ1
)
XCO (%)
SROH (%)
C2+OHb (%)
Cu–Co/Al2O3 (ref. 20)
Co–La/AC48
Cu–Co/La2O3 (ref. 36)
Cu–Co/La2O3–ZrO2 (ref. 27)
Cu–Co/La2O3–SiO2 (ref. 26)
250
225
275
310
330
300
300
2
2
2
2
2
2
2
2
3
6.89
3
3
1800c
500
5000
3900c
3900c
3900c
3900c
23.2
58
23.3
20.4
40.5
43.4
39.4
39.3
37.0
79.3
88.8
60
82.3
66.1
86.1
79.5
15.7
35.3
32.1
50.9
47.1
d
Cu–Co/La2O3–LaFeO3
Cu–Co/La2O3–LaFeO3
3
3
e
The molar ratio of H2/CO. b The mass fraction of C2+OH including ethanol in the total alcohols. c The unit is mL gcatꢀ1 hꢀ1
work in stability test.
.
In this work. e This
a
d
were changed to nanoparticles of metal copper and Co2C
(Fig. 2c).
4.2. Catalytic performance
The prepared Cu–Co/La2O3–LaFeO3 (M) catalysts showed good
activity, excellent selectivity and very good stability for HAS from
syngas as shown in Section 3.5. The excellent performance for
these catalysts can be owned to the following reasons.
The ions of copper and cobalt can act as the crystalline ions
of the PTO of LaFeO3, which means that the copper and cobalt
species should be interacted with LaFeO3 and La2O3. The
interaction between copper and cobalt with LaFeO3 and La2O3
should exist and strong, which kept the uniform mixing of the
elements in the catalyst during reaction (Fig. 5b and b0).
(2) As can be seen in Fig. 1, the N2 adsorption–desorption
isotherms and the corresponding pore size distribution of 6%
Co3O4/LaFe0.7Cu0.3O3 (M) and 6% Co3O4/LaFe0.7Cu0.3O3 (M)
aer 200 hours' stability test show the similar trends, suggest-
ing that the pore structures of 6% Co3O4/LaFe0.7Cu0.3O3 (M) are
stable in the reaction conditions.
In Fig. 4b–d, Cu–Co alloy, LaFeO3 and La2O3 are uniformly
mixed in the catalysts, and those compounds are in nano size.
Thus, the agglomerating and sintering of the same kind of
nanoparticles was blocked by the other nanoparticles, such as
the agglomerating and sintering of LaFeO3 would be blocked by
the species or nanoparticles of copper, cobalt and La2O3 (Fig. 4f,
h and Table 5).
Firstly, according to the XRD and TEM results, Cu–Co alloy
was obtained by the reduction of Co3O4/LaFe0.7Cu0.3O3. It is
general accepted that metal copper is the active site for the
formation of alcohols, while metal cobalt for the C–C chain
growth, the copper and cobalt work cooperatively to generate
higher alcohols.18 Based on this mechanism, it is recognized
that the formation of Cu–Co alloy is much important for the
catalytic performance of Cu–Co catalysts for HAS.19–21 At the
same time, the formation of Cu–Co alloy favors the formation of
Co2C,19 and the formation of Co2C favors the synthesis of higher
alcohols.47,48
Secondly, aer reduction of LaFe0.7Cu0.3O3, the side product
of La2O3 can be acted as a promoter for higher alcohols
synthesis. Jiao et al.48 and Lebarbier et al.49 reported that La2O3
can promote the formation of Co2C and optimize the propor-
tion of Co2+ to metal cobalt, while the formation of Co2C favors
the formation of higher alcohols, hence the selectivity to higher
alcohols can be improved. Seen from Fig. 4b–d, La2O3 generated
from the reduction of LaFe0.7Cu0.3O3 are mixed uniformly with
the bimetallic Cu–Co alloy particles, for that copper and
lanthanum species in the precursor of LaFe0.7Cu0.3O3 are
uniformly mixed. The uniform mixture of La2O3 with Cu–Co
alloys should favor the promotion effect of La2O3 working.
Thirdly, the mesopores of Cu–Co/La2O3–LaFeO3 (M) catalyst
may play a favorable role on the higher alcohols synthesis. Ding
et al.50 found that Cu–Fe 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 the products inside the bimodal
porous structures. Lu et al.51 found that the unsupported three-
dimensionally ordered macroporous Cu–Fe 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 c-Fe5C2. In Cu–Co/La2O3–LaFeO3 (M), the
mesoporosity and the high surface area may favor the well
dispersion of the metal and the high diffusion efficiency of the
(3) For 6% Co3O4/LaFe0.7Cu0.3O3 which has no mesoporosity,
the elements are also uniformly mixed, and the sizes of nano
crystal lines are only a little larger (about ten percent) than that
of the catalysts with mesoporosity (Table 5). However, the nano
crystals incline to agglomerated into much large particles
(Fig. 4g), which means that the active components of copper
and cobalt would be stacked. This is harmful for the catalyst.
Table 5 Crystal sizesa (nm) of the catalysts after reduction and
reaction
Aer reduction Aer reaction
Cu–Co
Samples
LaFeO3 alloy
LaFeO3
Cu
10% Co3O4/LaFe0.7Cu0.3O3 (M) 10.3
8.3
9.6
9.0
9.5
11.4
12.6
8.7
9.4
6% Co3O4/LaFe0.7Cu0.3O3
6% Co3O4/LaFe0.7Cu0.3O3 (M)
2% Co3O4/LaFe0.7Cu0.3O3 (M)
11.9
9.3
11.5 (7.1b) 7.9 (8.5b)
9.0
9.0
9.5
a
b
Calculated by X-ray diffraction with Scherrer equation. Data of the
catalysts aer stability test.
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