Remarkably efficient CoGa catalyst with uniformly dispersed and trapped structure for ethanol and higher alcohol synthesis from syngas

Article abstract of DOI:10.1016/j.jcat.2016.05.014

Ethanol and higher alcohols (C₂₊ alcohols) used as liquefaction fuels and/or feedstocks for chemicals could be of great significance if they could be efficiently synthesized from the transformation of biomass-based syngas. Here, we report a uniformly dispersed CoGa catalyst with CoGa particles trapped in the oxide support, which is not only highly active and selective but also distinctively stable in the synthesis of ethanol and higher alcohols from syngas. A 43.5% CO conversion with an alcohol selectivity of 59% has been achieved. In the alcohol products, the fraction of ethanol and higher alcohols reaches 93%. More significantly, the trapped CoGa shows no visible changes of particle dispersion and homogeneous CoGa distribution in the reaction, which gives rise to stable catalytic performance.

Full text of DOI:10.1016/j.jcat.2016.05.014

Journal of Catalysis 340 (2016) 236–247
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Remarkably efficient CoGa catalyst with uniformly dispersed and
trapped structure for ethanol and higher alcohol synthesis from syngas
⇑
Xun Ning, Zhe An, Jing He
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Ethanol and higher alcohols (C2+ alcohols) used as liquefaction fuels and/or feedstocks for chemicals
could be of great significance if they could be efficiently synthesized from the transformation of
biomass-based syngas. Here, we report a uniformly dispersed CoGa catalyst with CoGa particles trapped
in the oxide support, which is not only highly active and selective but also distinctively stable in the
synthesis of ethanol and higher alcohols from syngas. A 43.5% CO conversion with an alcohol selectivity
of 59% has been achieved. In the alcohol products, the fraction of ethanol and higher alcohols reaches 93%.
More significantly, the trapped CoGa shows no visible changes of particle dispersion and homogeneous
CoGa distribution in the reaction, which gives rise to stable catalytic performance.
Received 31 March 2016
Revised 13 May 2016
Accepted 15 May 2016
Keywords:
Ethanol and higher alcohols
CoGa particles
Syngas transformation
Layered double hydroxides
Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
However, a sole Co site has low selectivity toward alcohols because
of the deficiency of CO nondissociation sites. One of the approaches
Catalytic conversion of synthesis gas (syngas, CO/H2) to ethanol
and higher (C2+) alcohols has drawn much interest [1–6] owing to
the potential use of alcohols as alternative fuels and chemical feed-
stocks [7–9]. In addition to coal gasification [10,11] and natural gas
oxidation [12–14], syngas can be efficiently manufactured from
renewable dry biomass [15,16]. The catalytic synthesis of ethanol
and higher alcohols from syngas is a reliable method of energy
and fuel production independent of fossil resources. The synthesis
of C2+ alcohols from syngas includes both carbon-chain growth via
C–C coupling and alcohol production through CO insertion fol-
lowed by hydrogenation [17,18]. The kinetic rate match between
C–C coupling and CO insertion could promote the production of
C2+ alcohols, which competes with the formation of methanol
and hydrocarbons.
Several catalyst systems have been developed for syngas
conversion to ethanol and higher alcohols. Rh is the only single
metal able to selectively produce ethanol or C2+ oxygenates from
CO hydrogenation [19–21]. However, the high cost and limited
availability of Rh restrict its large-scale application in syngas
conversion. As an alternative, modified Fischer–Tropsch catalysts
based on Co and Fe [22–24], as well as Mo-based [25–27] catalysts,
have been proposed for C2+ alcohol production from syngas.
Because Co has prominent chain growth ability [28] and lower
activity toward the water–gas shift (WGS) [29,30], Co-based
catalysts have been attracting renewed scientific interest.
is to build bimetallic catalyst systems, such as CoPd [31] or CoCu
[32–38]. The selectivity to C2+ alcohols is thereby enhanced, as
CO adsorption on Pd0 or Cu0 is associative rather than dissociative.
Moreover, the addition of La2O3 dramatically enhances the
selectivity to C2+ alcohols over Co/AC catalysts [39,40], which is
attributed to the generation of Co2C, which promotes associative
Co adsorption [41]. But it is still a great challenge to enhance the
selectivity toward C2+ alcohols while retaining the activity.
Another challenge arises from the poor stability of Co-based cata-
lysts, which generally results from the sintering of the active
phase. The agglomeration could also lead to phase separation,
which is regarded as the main cause for the decrease in selectivity
to alcohols or higher alcohols [42–45]. Except for the report that
the copper and cobalt located in the well-defined perovskite struc-
ture with a slit-shaped space between nanoparticles improve the
stability [46], this remains an issue until now.
Here we report an efficient CoGa catalyst for syngas conversion
to ethanol and higher alcohols, in which the CoGa components are
uniformly dispersed and the CoGa particles are trapped on the sup-
port surface from the in situ transformation of layered double
hydroxides (LDHs) (Scheme 1). LDHs, unique two-dimensional
nanostructured materials with metal cations distributed in a
highly ordered manner in brucite-like layers [47], are an effective
precursor for supported metal catalysts with high-density catalytic
sites well dispersed [48–51]. The CoGa catalyst prepared in this
work displays a selectivity of more than 90% to ethanol and higher
alcohols in total alcohol products, with a CO conversion of 43.5%,
and especially a stable catalytic performance in the reaction.
⇑
Corresponding author. Fax: +86 10 64425385.
E-mail address: jinghe@263.net.cn (J. He).
http://dx.doi.org/10.1016/j.jcat.2016.05.014
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
237
Scheme 1. Schematic illustration for the preparation of uniformly dispersed and trapped CoGa particles from LDHs precursors.
For controls, Co2+/ZnGaAl-LDHs/
ZnAl-LDHs/ -Al2O3 with a Co/Ga molar ratio of 5/3 were prepared
using the incipient wetness impregnation method. The
as-synthesized ZnGaAl-LDHs/ -Al2O3 or ZnAl-LDHs/ -Al2O3 was
c /
-Al2O3 and Co2+Ga3+
2. Experimental
2.1. Preparation
c
c
c
impregnated with a solution of Co(NO3)2ꢀ6H2O or a mixed solution
of Co(NO3)2ꢀ6H2O and Ga(NO3)3ꢀxH2O. After impregnation for 4 h
at room temperature, the solid was dried at 100 °C for 12 h.
CoZnGaAl-LDHs with
synthesized in situ on the surface of spherical
affording a CoZnGaAl-LDHs/
Co(NO3)2ꢀ6H2O, 0.2678 g of Zn(NO3)2ꢀ6H2O, 0.0345 g of
Ga(NO3)3ꢀxH2O, and 0.2403 g of urea were first dissolved in 1.5 mL
of deionized water. The resulting mixed solution was transferred
a
Co/Ga molar ratio of 5/3 were
-Al2O3 [52],
-Al2O3 sample. Typically, 0.0873 g of
c
c
CoGa–ZnAl-LDO/
tion of CoZnGaAl-LDHs/
-Al2O3 was put into a quartz boat, which was placed at the center
c
-Al2O3 was prepared through in situ reduc-
c-Al2O3. About 1 g of CoZnGaAl-LDHs/
c
of a horizontal quartz tube inserted into a furnace at atmospheric
pressure. After the removal of air, the furnace was heated under
flowing H2 (40 mL/min) at 5 °C/min to 700 °C. The reduction was
maintained for 2 h at 700 °C. After the furnace was cooled to room
temperature, 1% O2–N2 was introduced into it for another 1 h. The
to a 15 mL autoclave. Then 1.0 g of
c-Al2O3 (20–40 mesh) was
added to the solution as both substrate and Al source. After being
kept shaking in a table concentrator for 2 h at room temperature,
the autoclave was tightly sealed and maintained at 100 °C for
another 12 h. The resulting solid was washed thoroughly with
deionized water and then dried at 60 °C for 12 h. CoZnAl-LDHs/
control samples, Co–ZnAl-LDO/
and CoGa/ZnAl-LDO/ -Al2O3, were prepared following the same
process by reduction of CoZnAl-LDHs/
-Al2O3, Co2+/ZnGaAl-LDHs/
-Al2O3, and Co2+Ga3+/ZnAl-LDHs/
-Al2O3. It should be noted that
c-Al2O3, Co/ZnGaAl-LDO/c-Al2O3,
c
c
-Al2O3, ZnGaAl-LDHs/
synthesized following the same process. The specific surface area
of bare
-Al2O3 is 150.3 m2/g, and the specific surface areas for
CoZnGaAl-LDHs/ -Al2O3, CoZnAl-LDHs/ -Al2O3, ZnGaAl-LDHs/
-Al2O3, and ZnAl-LDHs/
-Al2O3 are 186.1 m2/g, 175.4 m2/g,
180.7 m2/g, and 165.3 m2/g, respectively.
c-Al2O3, and ZnAl-LDHs/c-Al2O3 were all
c
c
c
c
the catalysts used for characterizations were passivized with pure
N2 instead of 1% O2–N2 after the furnace was cooled to room tem-
perature, and preserved under N2. The Co and Ga content were
c
c
c
c
Fig. 1. (A) XRD patterns of (a)
c-Al2O3, (b) CoZnGaAl-LDHs/c-Al2O3, (c) CoZnAl-LDHs/c-Al2O3, (d) ZnGaAl-LDHs/c-Al2O3, and (e) ZnAl-LDHs/c-Al2O3. (B) SEM image of
CoZnGaAl-LDHs/
CoZnGaAl-LDHs/
c
c
-Al2O3 with cross-sectional image inset. (C) SEM–EDS elemental mapping images for Co, Ga, and Zn of the corresponding region (yellow frame in B) of
-Al2O3.

238
X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
nanometers to eventually prevent magnetization. The
temperature-programmed reduction (H2 TPR) of the catalyst pre-
cursor was performed on
chemisorption system with
a
a
Micrometric ChemiSorb 2720
thermal conductivity detector
(TCD). The sample (ꢁ0.1 g) was pretreated in a flow of Ar
(40 mL/min) at 150 °C for 1 h to remove the water and then cooled
to 30 °C. An H2 TPR profile was recorded from 30 to 900 °C at a
heating rate of 10 °C/min in a flow of 10% H2–Ar (40 mL/min).
The temperature-programmed desorption of H2 (H2 TPD) on each
catalyst was carried out on the same equipment. The sample
(ꢁ0.1 g) was pretreated in a flow of 10% H2–Ar (40 mL/min) at
400 °C for 1 h, cooled to 100 °C, held at 100 °C for 1 h for H2 absorp-
tion, and then purged by a flow of Ar (40 mL/min). The H2 TPD pro-
file was recorded from 100 to 450 °C at a heating rate of 10 °C/min
and held at 450 °C for 2 h in a flow of Ar (40 mL/min). Calculations
for Co dispersion were made using the total amount of adsorbed
hydrogen and a stoichiometry of one hydrogen atom per cobalt
surface atom. The dispersion was also determined from TEM
images by the particle mean size, assuming a surface atom density
of 14.6 Co atoms/nm2 [53]. The Co K-edge X-ray absorption near-
edge structure (XANES) spectra were recorded in transmission
mode at room temperature at beam line 1W1B of at the Beijing
Synchrotron Radiation Facility (BSRF), Institute of High Energy
Physics, Chinese Academy of Science. A Si (III) double crystal was
used to monochromatize the X-rays from the 700 MeV electron
storage ring. XANES data were examined by Athena program.
2.3. Catalytic test
The catalytic test was performed in a stainless steel fixed-bed
reactor. A portion of 0.6 g of catalyst (20–40 mesh) was loaded into
the reactor. The remaining volume of the reactor tube was filled
with quartz beads of 20–40 mesh. Prior to the reaction, the catalyst
was activated in situ with H2 at atmospheric pressure with a flow
rate of 2000 hꢂ1 at 40 °C for 1 h. After the reactor was cooled to
50 °C, the reactor was heated to the reaction temperature with a
ramp rate of 2 °C/min and a syngas (H2/CO = 2.0, 5% argon as inter-
nal standard) flow rate of 2000 hꢂ1. When the reaction tempera-
ture was reached, the system was pressurized to 3.0 MPa with
syngas. The reaction was conducted at 240, 260, 280, and 300 °C.
After passing through a hot trap (180 °C) and a cold trap (0 °C),
the tail gas was analyzed online by GC. Ar, CO, CH4, and CO2 were
analyzed through a TDX-1 packed column with a thermal conduc-
tivity detector (TCD), using He as carrier gas (Fig. S1 in the Supple-
mentary Information). C1–C5 hydrocarbons were analyzed through
an Al2O3 packed column with N2 carrier and hydrogen flame ioniza-
tion detector (FID) (Fig. S2). The aqueous and oil liquid products
(Table S1), collected from the cold trap and hot trap, were analyzed
offline by GC. The aqueous products containing alcohols mostly
Fig. 2. (A) XRD patterns of (a) CoGa–ZnAl-LDO/
c
-Al2O3, (b) Co–ZnAl-LDO/
c
-Al2O3,
-Al2O3,
(c) Co/ZnGaAl-LDO/
(a) CoZnAl-LDHs/
-Al2O3, (b) Co2+/ZnAl-LDHs/
(d) Ga3+/ZnAl-LDHs/
-Al2O3, (e) CoZnGaAl-LDHs/
-Al2O3, and (g) Co2+Ga3+/ZnAl-LDHs/
-Al2O3.
c
-Al2O3, and (d) CoGa/ZnAl-LDO/
c
-Al2O3. (B) H2 TPR profiles of
c
c
-Al2O3, (c) ZnGaAl-LDHs/c
c
c
-Al2O3, (f) Co2+/ZnGaAl-LDHs/
c
c
determined by ICP-ES as 1.57 and 1.06 wt.% for CoGa–ZnAl-LDO/
-Al2O3, 1.62 and 1.03 wt.% for Co/ZnGaAl-LDO/ -Al2O3, and 1.59
and 1.05 wt.% for CoGa/ZnAl-LDO/ -Al2O3, respectively. The Co
-Al2O3 was determined to be 1.53 wt.%.
c
c
c
content in Co–ZnAl-LDO/
c
2.2. Characterizations
X-ray diffraction (XRD) measurements were carried out on a
Shimadzu XRD-6000 powder diffractometer with Cu K radiation
(k = 0.154 nm) operated at 40 kV and 30 mA. The XRD patterns
for all the samples were collected with a scanning angle (2h) range
of 3–70° at a scanning rate of 5°/min. The quantitative elemental
a
were analyzed through GSBP-INOWAX (30 m ꢃ 0.25 mm ꢃ 0.5
lm)
with an N2 carrier and FID. Sec-butyl alcohol was used as an internal
standard (Fig. S3). The liquid oil products were diluted in cyclohex-
ane solvent and analyzed through a DB-5 column with N2 carrier
and FID. Ethyl cyclohexane was used as an internal standard
analysis for Co and Ga was performed using
a Shimadzu
ICPS-75000 inductively coupled plasma emission spectrometer
(ICP-ES). N2 adsorption/desorption experiments were carried out
on a Quantachrome Autosorb-1 system. The specific surface area
was determined by Brunauer–Emmett–Teller (BET) methods using
a Quantachrome Autosorb-1C-VP analyzer. The samples were out-
gassed in N2 flow at 120 °C for 2 h prior to measurement. The SEM
(Fig. S4). CO conversion (
vCO) and selectivity of products were cal-
culated according to the following equations:
F
ꢂ F
COin
COout
vCO
¼
ꢃ 100%
F
COin
images were taken on
a Zeiss Supra 55 scanning electron
FC ꢃ i
i
microscope equipped with an energy-dispersive spectrum (EDS)
attachment. TEM and HRTEM images were taken on a Tecnai G2
F20 S-TWIN operated at 300 kV. High-angle annular dark field
(HAADF) images and elemental analysis (mapping and line scan)
were taken in STEM mode. The samples for STEM measurements
were treated in ethanol under ultrasonic conditions before deposi-
tion on a Cu microgrid and covered with a carbon coating of several
SC
¼
F
ꢃ 100%
i
ꢂ F
COin
COout
n
X
SROH
¼
SC H
;
2iOH
i
i¼1
where F is the moles of CO and product Ci (CO2, hydrocarbon or
alcohols) containing i carbon atoms. The mass balance and carbon

X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
239
Fig. 3. TEM images, HRTEM images, and particle size distributions for (A) CoGa–ZnAl-LDO/
c
-Al2O3, (B) Co–ZnGaAl-LDO/
c
-Al2O3, (C) Co/ZnGaAl-LDO/
c
-Al2O3, and (D) CoGa/
ZnAl-LDO/c-Al2O3. The estimated mean sizes and dispersions (D) are given with the particle size distributions. The particle size distributions and mean sizes were based on
250 particles counted in various regions. The dispersions were determined by the particle mean sizes assuming a surface atom density of 14.6 Co atoms/nm2 [53]. The
dispersions determined by H2 chemisorption are shown in parentheses.
balance have been calculated at each product and kept between 90%
and 95%.
[52]. For comparison, CoZnAl-LDHs, ZnGaAl-LDHs, and ZnAl-LDHs
were also synthesized on -Al2O3 as well. In the X-ray diffraction
(XRD) patterns (Fig. 1A), in addition to the reflections characteristic
of -Al2O3 at 37.42°, 39.25°, 45.84°, and 67.08° (JCPDS: 10-0425),
the (003), (006), (009), (110), and (113) reflections at about
11.7°, 23.6°, 35.4°, 60.3°, and 61.2° characteristic of
c
c
3. Results and discussion
a
3.1. Supported CoGa catalysts
hydrotalcite-like structure are clearly observed in each LDH-
grown sample. The basal spacing is estimated to be nearly
0.73 nm, illustrating that the interlayer anion is CO23ꢂ. The lattice
parameter a (a = 2d(110)) for CoZnGaAl-LDHs (a = 1.5288) is larger
than that for CoZnAl-LDHs (a = 1.5268), resulting from the greater
3.1.1. Dispersion of CoGa particles
CoZnGaAl-LDHs with a Co/Ga molar ratio of 5/3, as the precur-
sor for preparation of CoGa supported on ZnAl-LDO, were synthe-
sised in situ on the c-Al2O3 support using urea as the precipitant

240
X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
Fig. 4. Line scan profiles of Zn and Al population (full lines) and Co population (dashed lines) in (A) CoGa–ZnAl-LDO/c-Al2O3 and (B) Co/ZnGaAl-LDO/c-Al2O3 to show the
stabilization of CoGa particles anchored by the traps in the LDO supports transformed CoZnGaAl-LDHs.

X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
241
Fig. 5. HAADF images with Co (red) and Ga (green) element mapping and linescan profiles along the yellow line for (A) CoGa–ZnAl-LDO/
c
-Al2O3, (B) Co/ZnGaAl-LDO/
c
-Al2O3,
and (C) CoGa/ZnAl-LDO/c-Al2O3.
Ga–O length (1.92 Å) than Al–O length (1.91 Å).
A
similar
After thermal treatment of CoGa–ZnAl-LDHs/c-Al2O3 or
expansion of the lattice parameter a is also observed for ZnGaAl-
LDHs (a = 1.5249) by comparison with ZnAl-LDHs (a = 1.5224),
which proves the substitution of Ga3+ for Al3+ in the LDH brucite-
like layers. From the SEM images (Fig. 1B), the CoZnGaAl-LDHs
are grown densely on both exterior and interior surfaces of
Co–ZnAl-LDHs/
LDO/ -Al2O3 or Co–ZnAl-LDO/
ison, Co/ZnGaAl-LDO/ -Al2O3 and CoGa/ZnAl-LDO/
Co and/or Ga loadings similar to those on CoGa–ZnAl-LDO/
c
-Al2O3 at 700 °C under H2 for 2 h, CoGa–ZnAl-
-Al2O3 was produced. For compar-
-Al2O3 with
c
c
c
c
c
-Al2O3 were prepared by thermally treating Co/ZnGaAl-LDHs/
-Al2O3 and CoGa/ZnAl-LDHs/ -Al2O3 at 700 °C under H2 for 2 h,
-Al2O3 or CoGa/ZnAl-LDHs/
-Al2O3 was prepared by impregnation of ZnGaAl-LDHs/ -Al2O3
-Al2O3 with Co2+ or Co2+/Ga3+ salts. In the XRD pat-
-Al2O3, the ZnAl-
LDO phase can be clearly seen in each reduced sample according
c-Al2O3, displaying a thickness of approximately 1.84
lm. The
c
c
SEM–EDS element mapping images (Fig. 1C) offer clear evidence
for the homogeneous distribution of Co, Ga, and Zn elements in
in which the Co/ZnGaAl-LDHs/c
c
c
the LDH layers. In comparison with bare
grown samples exhibit increased specific surface area due to the
orientation growth of LDHs on the -Al2O3 support.
c
-Al2O3, all the LDH-
or ZnAl-LDHs/c
terns (Fig. 2A), in addition to the reflections of
c
c

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X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
(111) facet assigned to CoGa particles can be clearly observed,
with a lattice spacing of 0.210 nm. The CoGa particles exhibit
extraordinarily excellent dispersion with a narrow size distribution
from 6 to 14 nm and a mean size of 8.5 nm. The dispersion is 12.9%
(12.3%). For Co–ZnAl-LDO/c-Al2O3 (Fig. 3B), the HRTEM image
shows the (111) facet of Co particles, with a lattice spacing of
0.202 nm. The particle size ranges between 3 and 10 nm (mean
size = 6.7 nm) and the dispersion is 16.3% (15.6%). However, in
Co/ZnGaAl-LDHs/c-Al2O3, which is produced from ZnGaAl-LDHs
with Co salts impregnated, the lattice spacing of the (111) facet
is estimated to be 0.204 nm (Fig. 3C). The particle size ranges from
4 to 12 nm (mean size = 7.6 nm) and the dispersion is 14.4%
(14.1%). For CoGa/ZnAl-LDO/c-Al2O3 (Fig. 3D), which is produced
from the ZnAl-LDHs with both Co and Ga salts impregnated, the
particle size ranges from 5 to 20 nm (mean size = 12.3 nm) and
the dispersion is 8.9% (7.8%). The lattice spacing of the (111) facet
is estimated to be 0.207 nm, similar to that for CoGa particles.
Therefore, the insertion of Co and/or Ga cations into the lattices
of LDH layers facilitates Co or CoGa particles with small size and
narrow size distribution. It is obvious that the lattice spacing
Fig. 6. The normalized intensity of Co K-edge XANES spectra of Co foil, CoGa–ZnAl-
LDO/c-Al2O3, Co–ZnAl-LDO/c-Al2O3, Co/ZnGaAl-LDO/c-Al2O3, and CoGa/ZnAl-LD O/
increases in the order Co–ZnAl-LDO/
c-Al2O3 < Co/ZnGaAl-LDO/
c
-Al2O3.
c
-Al2O3 <CoGa/ZnAl-LDO/ -Al2O3 <CoGa–ZnAl-LDO/c
c
-Al2O3, which
visibly depends on whether and how Ga has been introduced. The
difference in the lattice spacing might be relevant to the Co and Ga
distribution, which is to be discussed hereafter.
to the reflections at 32.17° and 37.52° (JCPDS: 05-0669). The
diffractions attributed to Co or CoGa (Co JCPDS: 15-0806, 2h
(111) = 44.2°, d(111) = 0.202 nm; CoGa JCPDS: 15-0578, 2h(111)
= 42.8°, d(111) = 0.210 nm) are difficult to identify because they
are close to those of the (400) reflection of c-Al2O3, or the metal
3.1.2. Trapped accommodation of CoGa particles
phase exists in a highly uniform dispersion. The presence of Co
or CoGa is revealed from the H2 TPR of the catalyst precursor. As
shown in Fig. 2B, the H2 consumptions are observed at 569 and
From the TEM images (Fig. 3A) of CoGa–ZnAl-LDO/c-Al2O3, the
CoGa particles are clearly observed to be trapped in the ZnAl-LDO.
To confirm the trapped structure, Zn and Al populations in the LDO
support are counted. As shown in Fig. 4A, the scanned Zn and Al
populations corresponding to the inset TEM image shows a peri-
odic decrease/increase, and the decrease position exactly matches
the presence of CoGa particles. In virtue of the unique structure of
LDH materials, the elements Co, Ga, Zn, and Al are all distributed in
a highly ordered manner in the LDH layers. During the topological
transformation/in situ reduction of LDHs to LDOs and metal parti-
cles, Co and Ga are reduced to form CoGa particles and Zn and Al
are transformed to oxides. The traps for the metal particles are
generated where Zn and Al are absent. Obviously, the observed
traps accommodate the CaGa particles, which can be expected to
suppress the migration and aggregation of CoGa particles effec-
680 °C for CoZnAl-LDHs/c-Al2O3 and at 525 and 611 °C for
ZnGaAl-LDHs/c-Al2O3. The H2 consumption between 500 and
700 °C is attributed to the reduction of Co(II) to Co0 [54] and Ga
(III) to Ga0 [55]. Notably, the temperatures of H2 consumption for
CoZnAl-LDHs/
than those for Co2+/ZnAl-LDHs/
LDHs/ -Al2O3 (361 and 390 °C), indicating that the reduction of
Co(II) or Ga(III) confined in the LDH layers is more difficult. In com-
parison with CoZnAl-LDHs/ -Al2O3, the temperatures of H2 con-
sumption for CoGaZnAl-LDHs/ -Al2O3 decrease by 30 °C. This is
indicative of an interaction between Co and Ga, revealing the
formation of CoGa particles. For Co2+/ZnGaAl-LDHs/
-Al2O3 or
Co2+Ga3+/ZnAl-LDHs/
-Al2O3, the temperature of H2 consumption
c
-Al2O3 or ZnGaAl-LDHs/
c-Al2O3 are much higher
c
-Al2O3 (269 °C) or Ga3+/ZnAl-
c
c
c
c
c
tively. For Co/ZnGaAl-LDO/c-Al2O3, which is produced from the
decreases by 13 or 21 °C, suggesting the weakened Co–Ga
interaction.
Visible particles can be observed in the TEM images (Fig. 3).
From the HRTEM image of CoGa–ZnAl-LDO/c-Al2O3 (Fig. 3A), the
Co-salt-impregnated ZnGaAl-LDHs, the decrease/increase of Zn
and Al populations is not always visible, and the positions of
population decrease (traps) are not observed to accommodate
the CoGa particles (Fig. 4B). The elements Zn, Ga, and Al are all
Table 1
Catalytic performance of CoGa–ZnAl-LDO/
c
-Al2O3, Co–ZnAl-LDO/
c
-Al2O3, Co/ZnGaAl-LDO/
c
-Al2O3, and CoGa/ZnAl-LDO/
c
-Al2O3 for synthesis of ethanol and higher alcohols from
Alcohols distribution (wt.%)d
syngas.a
b
Catalyst
Reaction time (h)
TOF (10ꢂ3 sꢂ1
)
CO conversion (%)
Selectivity (C mol%)c
CH4
C2+
H
ROH
CO2
MeOH
EtOH
PrOH
BuOH
C5+OH
CoGa–ZnAl-LDO/
c
-Al2O3
15
2
37.9
43.5
15.8
27.3
32.9
27.0
32.5
15.4
10.9
9.5
8.9
29.0
24.6
27.6
28.3
54.4
32.7
33.2
59.0
64.6
61.0
59.8
29.6
50.5
51.6
1.2
3.9
2.5
2.2
1.9
2.0
1.4
7.2
6.7
6.7
30.8
25.6
29.9
29.8
28.5
40.4
24.0
14.3
12.9
14.3
13.4
12.6
12.8
11.1
10.1
9.8
9.6
10.0
9.2
11.8
10.7
37.7
45.0
39.5
39.2
25.1
21.2
41.4
3.3
4.7
15
15
15
9.7
7.6
Co–ZnAl-LDO/
c
-Al2O3
29.4
31.8
34.2
14.1
14.8
13.8
24.6
13.8
12.7
Co/ZnGaAl-LDO/
CoGa/ZnAl-LDO/
c
c
-Al2O3
-Al2O3
a
b
c
Reaction conditions: T = 260 °C, P = 3 MPa, n(H2)/n(CO) = 2.0, GSHV = 2000 hꢂ1
TOF values are based on the dispersion estimated from the TEM images.
Carbon selectivity is defined as the selectivity of all the carbon-containing products from converted carbon; C2+H = hydrocarbons exclusive of methane; ROH = total 1-
.
alcohols.
d
Alcohol distribution (wt.%): the weight fraction of each alcohol in the total alcohols.

X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
243
Fig. 7. Anderson–Schulz–Flory (A–S–F) plots for the distribution of alcohols and hydrocarbons obtained at 15 h reaction for (A) CoGa–ZnAl-LDO/
c
-Al2O3, (B) Co–ZnAl-LDO/
c
ln
-Al2O3, (C) Co/ZnGaAl-LDO/
c
-Al2O3, and (D) CoGa/ZnAl-LDO/
, in which n is the number of carbon atoms in products, Wn is the weight fraction of products containing n carbon atoms, and 1 ꢂ
a-values are based on the confidence level over 96%.
c
-Al2O3. The A–S–F chain growth probability
a
of products is calculated according to the equation ln(Wn/n) = n
a
+ ln(1 ꢂ
a
)2/
a
a
is the probability of chain
termination. The
Table 2
CO conversion and alcohol selectivity in some representative catalysts reported in references.
Catalysts
T (°C)
H2/COa
P (MPa)
GHSV (hꢂ1
)
CO conversion (%)
SROH (%)
SC2+OH (%)
b
CoGa–ZnAl-LDO/
Cu–Co/ZrO2 [42]
c
-Al2O3
260
310
220
220
200
240
330
250
2
2
2
2
1
2
2
2
3
4.5
2
3
6
6
3
2
2000
3900
2000c
33.6d
2000
2000
3900c
1800c
43.5
60
21.5
71.4
12.9
18
59.0
55
92.8
NA
80.8
92.6
94.6
NA
Cu@(CuCo-alloy)/Al2O3 [37]
Co–Co2C/AC1 [41]
50.6
38.4
47.6
37.5
39.5
23.3
3DOM Cu2Fe1 [24]
Co1Cu1Mn1 [35]
20%Cu–Co/La2O3–SiO2 [59]
15Cu5Co/Al2O3 [34]
32.1
23.2
66.1
79.3
a
b
c
The mole ratio of H2 to CO.
Catalyst in this work.
The unit is mL (gcath)ꢂ1
The unit is mL/min.
.
d
distributed in a highly ordered manner in the LDH layers. In the
reduction of LDHs without reducible metal cations in the layers,
only –OHs are removed to make a rough surface. The generated
pits on the surface of ZnGaAl-LDO are too small to accommodate
the particles in nanoscale. The results offer evidence for the
Co, which indicates a contiguous distribution of Co and Ga
(Fig. 5A). But in Co/ZnGaAl-LDO/ -Al2O3, the Ga is observed to be
in a homogeneous and continuous distribution, while the Co is vis-
ibly distributed in islands (Fig. 5B). The inhomogeneous distribu-
tion of Co and Ga results in a (111) lattice of particles in Co/
c
trapped CoGa particles in CoGa–ZnAl-LDO/
to the confinement of CoGaZnAl-LDHs structure in the calcina-
tion/reduction from CoZnGaAl-LDHs.
c
-Al2O3, which is due
ZnGaAl-LDO/
ZnAl-LDO/ -Al2O3. Co and Ga distributed in islands are also
observed in CoGa/ZnAl-LDO/ -Al2O3 (Fig. 5C), but less homoge-
neous than for the CoGa–ZnAl-LDO/ -Al2O3 produced from
CoGaZnAl-LDHs/ -Al2O3 with both Co and Ga located in the LDH
c-Al2O3 similar to that of the Co particles in Co–
c
c
c
c
3.1.3. Distribution of Co and Ga
brucite-like lattices. Therefore, introducing Co and Ga in the same
way helps promote their contiguous distribution to form a CoGa
phase. By using CoGaZnAl-LDHs/c-Al2O3 as precursor, in which
Co and Ga cations are both introduced into the lattices of LDHs
The distribution of Co and Ga in CoGa–ZnGa-LDO/
ZnGaAl-LDO/ -Al2O3, and CoGa/ZnAl-LDO/ -Al2O3 was measured
by EDS element mapping and linescan technique (Fig. 5). For
CoGa–ZnGa-LDO/ -Al2O3 derived from CoZnGaAl-LDHs/ -Al2O3,
both Co and Ga islands are observed, with Ga overlapped with
c-Al2O3, Co/
c
c
c
c
in the formation of the LDH precursor, well-defined CoGa particles

244
X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
on CoGa–ZnAl-LDO/c-A2O3 reaches 43.5% at 15 h, with an alcohol
selectivity of 59%. Among the alcohol products, the fraction of
ethanol and higher alcohols is up to 92.8%. According to GC–MS
analysis results (see the Supplementary Information), the alcohol
products are composed of 1-alcohols with the carbon number
ranging up to C16. As expected, unwanted CO2 is under control
for all the four catalysts.
Mass transport and heat transfer calculations were performed
for CoGa–ZnAl-LDO/c-A2O3 with 43.5% CO conversion, using
Weisz–Prater and Mears analyses [56,57] (see the Supplementary
Information). The Weisz–Prater criterion,
ꢂr0AðobsÞ
q
cR2
DeCAs
CWP
¼
< 1;
gives 2.72 ꢃ 10ꢂ3 < 1, indicative of no internal diffusion limitations.
The Mears criterion,
ꢂr0AR2
1 þ 0:33cv
Fig. 8. (A) Profiles of CO conversion as a function of reaction temperature and (B)
alcohol selectivity and C2+ alcohol selectivity as functions of CO conversion on
<
;
CAbDe jn ꢂ cbbbjð1 þ 0:33n Þ
x
CoGa–ZnAl-LDO/
symbols).
c-A2O3 (square symbols) and Co/ZnGaAl-LDO/c-A2O3 (triangle
gives 3.17 ꢃ 10ꢂ6 < 1.0, indicating no interphase and intraparticle
heat transfer or mass transport limitations. Thus our catalytic sys-
tem is free from mass transport and heat transfer limitations.
3.2.1. Improved catalysis of well-distributed CoGa
As can be seen from Table 1, hydrocarbons are predominantly
yielded (up to 68.5%) on Co–ZnAl-LDO/c-Al2O3, with a conversion
of 27.0%. The selectivity to ROH is only 29.6%. The introduction of
Ga increases the selectivity of alcohols to 61% for a similar conver-
sion (27.3%). This obviously indicates that the presence of Ga
promotes CO insertion, which is considered the key step for the
formation of alcohols. Also, lower methanol selectivity and higher
C2+ alcohol fraction in total alcohol products, particularly C5+
alcohols, are observed on CoGa–ZnAl-LDO/
Co–ZnAl-LDO/ -Al2O3, proving that the introduction of Ga
additionally boosts the chain growth probability. Co/ZnGaAl-LDO/
-A2O3 affords lower alcohol selectivity (50.5%) than CoGa–
ZnAl-LDO/ -A2O3 (59.8%) for a similar conversion (32.9%). The
C2+ alcohol fraction is 86.2%. CoGa/ZnAl-LDO/ -A2O3 affords an
alcohol selectivity of 51.6%, which is improved in comparison to
Co/ZnGaAl-LDO/ -A2O3, while still lower than that on CoGa–
ZnAl-LDO/ -A2O3. The C2+ alcohol fraction on CoGa/ZnAl-LDO/
-A2O3 (87.3%) is slightly lower than that on CoGa–ZnAl-LDO/
-A2O3 (93.3%). The A–S–F plots of alcohols and hydrocarbons at
c-A2O3 than on
c
c
c
Fig. 9. CO conversion and ROH selectivity with reaction time on stream (100 h) on
c
CoGa–ZnAl-LDO/c-A2O3 (square symbols) and Co/ZnGaAl-LDO/c-A2O3 (triangle
symbols). The reaction was carried out at T = 260 °C, P = 3 MPa, GHSV = 2000 hꢂ1, n
(H2)/n(CO) = 2.0.
c
c
c
c
with a narrow size distribution have been produced. The well-
defined CoGa particles from CoGaZnAl-LDHs/c-Al2O3 are further
confirmed by XANES results (Fig. 6), in which the photoenergy
reflects the electronic structure of the Co center. The lowest energy
15 h are shown in Fig. 7. Accordingly, strictly linear A–S–F distribu-
tions are observed for alcohols and hydrocarbons over the entire Cn
range for each catalyst. The observation of the correlated a-values
is in agreement with a common mechanism of alcohol formation
of the Co K-edge is detected in CoGa–ZnAl-LDO/
of the greatest enrichment of electrons on the Co atom. This illus-
trates that the Co sites in CoGa–ZnAl-LDO/ -Al2O3 tend to be more
c-Al2O3, indicative
by CO insertion into the same type of intermediate for hydrocarbon
c
formation [24,39]. Obviously, CoGa–ZnAl-LDO/
c-A2O3 and CoGa/
negative, as the intimate contact of Co and Ga in the well-defined
CoGa particles facilitates electron transfer from Ga (electronegativ-
ity 1.6) to Co (electronegativity 1.88).
ZnAl-LDO/ -A2O3 exhibit similarly higher -values for alcohols
c
a
and hydrocarbons than the other two catalysts, indicating an
enhanced CO insertion ability accompanied by a boosted chain
lengthening probability. As resolved from the STEM results,
3.2. Syngas to ethanol and higher alcohols
CoGa–ZnAl-LDO/
tribution than CoGa/ZnAl-LDO/
tion of Co and Ga elements has been observed in Co/ZnGaAl-LDO/
-A2O3. Thus it can be concluded that more homogeneous distribu-
tion of Co and Ga elements promotes selective formation of not
only alcohol products but also C2+ alcohols. The intimate contact
between Co and Ga sites enhances CO insertion ability and also
chain growth probability, making the kinetic rate of CO insertion
and C–C coupling match to yield more C2+ alcohol products.
c
-A2O3 possesses more contiguous Co and Ga dis-
c
-A2O3, and no contiguous distribu-
CoGa–ZnAl-LDO/
conversion of syngas to ethanol and higher alcohols. For compar-
ison, catalysis by Co–ZnAl-LDO/ -A2O3, Co/ZnGaAl-LDO/ -A2O3,
and CoGa/ZnAl-LDO/ -A2O3 has also been evaluated. The
comparison is made in terms of CO conversion, selectivity, and A
nderson–Schulz–Flory (A–S–F) -chain-growth probability
(Table 1). To allow the selectivity comparison, the catalytic results
on CoGa–ZnAl-LDO/ -A2O3 are provided at varied CO conversions.
CoGa–ZnAl-LDO/ -A2O3 exhibits higher CO conversion and C2+
alcohol selectivity than the other three catalysts. CO conversion
c-A2O3 was then used as the catalyst for
c
c
c
c
a
c
In the same reaction time (15 h), CoGa–ZnAl-LDO/
vides higher CO conversion (43.5%) than Co–ZnAl-LDO/
(27.0%), demonstrating the role of Ga in boosting CO conversion.
c
-A2O3 pro-
c
c-A2O3

X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
245
Fig. 10. (A) TEM image with HRTEM image inset and (B) linescan profiles of Co, Ga, Zn, and Al elemental distribution of the used CoGa–ZnAl-LDO/
c
-Al2O3 after 100 h reaction.
(C) The particle size distributions for fresh and used CoGa–ZnAl-LDO/
CoGa–ZnAl-LDO/ -Al2O3.
c-Al2O3. (D) HAADF image with the corresponding Co (red) and Ga (green) element mappings in the used
c
CoGa–ZnAl-LDO/
tiguous distribution, exhibits higher CO conversion than Co/
ZnGaAl-LDO/ -A2O3 (32.5%), meaning that CO conversion is also
promoted by contiguous Co and Ga distribution. However, CoGa/
ZnAl-LDO/ -A2O3 provides much lower CO conversion (15.4%) than
CoGa–ZnAl-LDO/ -A2O3 in the same reaction time, although CoGa/
ZnAl-LDO/ -A2O3 possesses a contiguous Co and Ga distribution
similar to that for CoGa–ZnAl-LDO/ -A2O3. The CO conversion on
CoGa/ZnAl-LDO/ -A2O3 is even lower than that on Co/ZnGaAl-
LDO/ -A2O3 or Co–ZnAl-LDO/ -A2O3, suggesting that the particle
size and size distribution play an important role in determining
activity. Smaller particle size leads to higher catalytic activity.
Turnover frequencies (TOFs) allow activity comparison without
the influence of the particle size, as an earlier study [58] reveals
that Co nanoparticles larger than 6 nm have a similar TOF in the
Fischer–Tropsch synthesis. As listed in Table 1, the calculated
c
-A2O3, with Co and Ga in homogeneously con-
performance toward C2+ alcohol synthesis from syngas reported
in the literature are listed in Table 2. Obviously, the CoGa–ZnAl-
LDO/c-A2O3 catalyst turns into one of the catalysts having the
potential for scale-up in an industrial application to produce etha-
nol and higher alcohols, owing to the high selectivity to alcohols or
C2+ alcohols with a relatively high CO conversion.
For further study, reactions at different temperatures on
CoGa–ZnAl-LDO/c-A2O3 and Co/ZnGaAl-LDO/c-A2O3 were carried
out. Fig. 8 illustrates CO conversion as a function of reaction tem-
perature, as well as the selectivity to alcohols and fraction to C2+
alcohols versus CO conversion. Generally, increasing reaction
temperature results in a continuous increase in catalytic activity,
c
c
c
c
c
c
c
c
while CoGa–ZnAl-LDO/
Co/ZnGaAl-LDO/ -A2O3 at each temperature. A CO conversion of
40% is obtained at 256 °C on CoGa–ZnAl-LDO/ -A2O3, while
280 °C is required on Co/ZnGaAl-LDO/ -A2O3 to reach the same
conversion (Fig. 8A). In the case of CoGa–ZnAl-LDO/ -A2O3, the
c-A2O3 shows higher conversion than
c
c
c
TOF for CoGa–ZnAl-LDO/
c
-A2O3 is about 37.9 ꢃ 10ꢂ3 sꢂ1, a little
c
higher than that of the catalyst in the literature [58] under similar
experimental conditions (T = 250 °C, P = 3.5 MPa, TOF = 34.8 ꢃ
10ꢂ3 sꢂ1). With a similar contiguous Co and Ga distribution,
selectivity to alcohols reaches 63%, and 93% of the total alcohols
is C2+ alcohols (Fig. 8B) at 40% conversion. But in the case of Co/
ZnGaAl-LDO/c-A2O3, the selectivity to alcohols and the fraction
of C2+ alcohols are only 38% and 79%. The results further suggest
that the uniform Co and Ga dispersion of the CoZnGaAl-LDHs/
c-Al2O3 precursor improves the selective production of ethanol
and higher alcohols from syngas.
These results demonstrate that the presence of Ga in the CoGa
catalyst enhances both the activity and alcohol selectivity in
ethanol and higher alcohol synthesis from syngas, which can be
preliminarily understood in the following two ways. First, the
doping of Ga with contiguous distribution of Co contributes to
isolating more Co centers, on which associative CO adsorption is
favored. Thus, the associative CO insertion step is accelerated on
CoGa/ZnAl-LDO/
c
-A2O3 shows a TOF of 34.2 ꢃ 10ꢂ3 sꢂ1, which is
close to that for CoGa–ZnAl-LDO/c-A2O3. Relatively lower TOFs
are obtained on Co–ZnAl-LDO/
that is, without Ga, and
c
-A2O3 (TOF = 29.4 ꢃ 10ꢂ3 sꢂ1),
on Co/ZnGaAl-LDO/
c
-A2O3
(TOF = 31.8 ꢃ 10ꢂ3 sꢂ1), that is, without contiguous Co and Ga
distribution. It therefore turns out that the activity of Co sites in
syngas transformation is boosted by the intimate Ga sites in the
well-defined CoGa particles. With CoGa–ZnAl-LDO/c-A2O3 as cata-
lyst, in which Co and Ga are in homogeneously contiguous distri-
bution with CoGa particles narrowly sized between 6 and 14 nm,
a CO conversion of 43.5% and an alcohol selectivity of 59%, with
a fraction of 93% ethanol and higher alcohols, have been achieved.
For comparison, representative catalysts with excellent catalytic
CoGa–ZnAl-LDO/
c-A2O3, increasing the selectivity of alcohols. Sec-
ond, the intimate contact of Co and Ga in the well-defined CoGa

246
X. Ning et al. / Journal of Catalysis 340 (2016) 236–247
Fig. 11. (A) TEM image of the used Co/ZnGaAl-LDO/c-Al2O3 after 100 h reaction. (B) The particle size distributions for fresh and used Co/ZnGaAl-LDO/c-Al2O3.
particles facilitates electron transfer from Ga to Co. The adjacent
electron-rich Co centers are efficient for CO dissociative adsorption
and the subsequent C–C coupling, which enhances the CO conver-
sion/surface activity and the carbon chain growth. The unique
structural features of CoGa catalysts need further investigation
for the understanding of the role of Ga in the catalysis, which is
under way in our laboratory.
study of the promoting nature of the Ga component is going on
in our laboratory.
Acknowledgments
Financial support by National Nature Science Foundation of
China (NSFC), PCSIRT (IRT1205), the 973 Project (2011CBA00504,
2014CB932101), and the Fundamental Research Funds for the Cen-
tral Universities (YS1406) is gratefully acknowledged. J.H. particu-
larly appreciates the financial aid of the China National Funds for
Distinguished Young Scientists from the NSFC. This work is also
supported by the Beijing Engineering Center for Hierarchical
Catalysts.
3.2.2. Enhanced stability of trapped CoGa catalyst
The stability of CoGa–ZnAl-LDO/c-Al2O3 and Co/ZnGaAl-LDO/c-
Al2O3 was tested under medium reaction conditions for over 100 h.
The conversion of CO and the selectivity to alcohols as a function of
time on stream are presented in Fig. 9. After an initial induction
period, the CO conversion and ROH selectivity remain stable at
approximately 43% and 60% on CoGa–ZnAl-LDO/
c-Al2O3. By con-
Appendix A. Supplementary material
trast, on Co/ZnGaAl-LDO/ -Al2O3, the CO conversion begins to
c
decrease after 40 h and gradually attains stabilization at a lower
activity of 20%. The selectivity to ROH also drops nearly to 40%, fol-
lowing the loss of activity. The stable catalysis of CoGa–ZnAl-LDO/
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2016.05.014.
c
-Al2O3 is assumed to result from the trapping effects of ZnAl-LDO
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