Core-shell Cu@(CuCo-alloy)/Al2O3 catalysts for the synthesis of higher alcohols from syngas

Article abstract of DOI:10.1039/c4gc01633e

The production of higher alcohols by the catalytic conversion of synthesis gas (CO + H₂) is one of the most promising approaches for the utilization of nonoil resources, in which bimetallic catalysts based on Cu and Fischer-Tropsch (FT) reaction active elements (e.g. Co, Fe, Ni) are efficient and cost-effective candidates. Herein, we demonstrate the fabrication of core-shell Cu@(CuCo-alloy) nanoparticles (NPs) embedded on a Al₂O₃ matrix via an in situ growth of CuCoAl-LDH nanoplatelets on aluminum substrates followed by a calcination-reduction process, and they serve as efficient catalysts toward CO hydrogenation to produce higher alcohols. The composition, particle size and shell thickness can be tuned by changing the Cu/Co molar ratio in the LDH precursors, and the best catalytic behavior was obtained over the Cu/Co (1/2) catalyst with a CO conversion of 21.5% and a selectivity (C₆₊ slate 1-alcohols) of 48.9%, which is superior to the traditional modified FT catalysts. XPS, in situ FTIR spectroscopy and HAADF-STEM revealed that the unique electronic and geometric interaction between Cu and Co in the Cu@(CuCo-alloy) NPs contributes to the significantly enhanced catalytic performances. In addition, the 3D hierarchical structure of the Cu@(CuCo-alloy)/Al₂O₃ catalyst facilitates mass diffusion/transportation as well as prevents hotspot formation, accounting for its stability and recyclability. The Cu@(CuCo-alloy)/Al₂O₃ catalyst with significantly improved catalytic behavior can be potentially used in CO hydrogenation to produce higher alcohols.

Full text of DOI:10.1039/c4gc01633e

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Green Chemistry
►
Green Chemistry
Paper
Core−Shell Cu@(CuCo-alloy)/Al O Catalysts for the Synthesis of
2
3
Higher Alcohols from Syngas
‡
a
‡b
a
a
a
a
a
a
Wa Gao, Yufei Zhao, Haoran Chen, Hao Chen, Yinwen Li, Shan He, Yingkui Zhang, Min Wei,*
a
a
David G. Evans and Xue Duan
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
2
Abstract:Production of higher alcohols from thecatalytic conversion of synthesis gas (CO + H ) is one of
the most promising approaches for theutilization of nonoil resources, in which bimetallic catalysts based
on Cu and Fischer–Tropsch (FT) reaction active elements (e.g., Co, Fe, Ni) are efficient and cost-
effective candidates. Herein, we demonstrate the fabrication of core−shell Cu@(CuCo-alloy)
1
1
2
0
5
0
2 3
nanoparticles (NPs) embedded on a Al O matrix via an in situ growth of CuCoAl-LDHs nanoplatelets on
aluminum substrates followed by a calcination-reduction process, which serve as an efficient catalyst
toward CO hydrogenation to producehigher alcohols. The composition, particle size and shell thickness
can be tuned by changing theCu/Co molar ratio in the LDHs precursors, and the best catalytic behavior
was obtained over the Cu/Co (1/2) catalyst with a CO conversion of 21.5% and a selectivity (C6+ slate 1-
alcohols) of 48.9%, which is superior to the traditional modified FT catalysts. TheXPS, in situ FTIR
spectroscopy and HAADF-STEM reveal that theunique electronic and geometric interaction between Cu
and Co in theCu@(CuCo-alloy) NPs give contribution to the significantly enhanced catalytic
performances. In addition, the3D hierachical structureof Cu@(CuCo-alloy)/Al
mass diffusion/transportation as well as prevents thehotspot formation, accounting for its stability and
recycleability. The Cu@(CuCo-alloy)/Al catalyst with significantly improved catalytic behavior can
be potentially used in CO hydrogenation to produce higher alcohols.
2 3
O catalyst facilitates the
2
O
3
1
. Introduction
syngas, bimetallic catalysts based on Cu and Fischer–Tropsch
FT) reaction active elements (e.g., Co, Fe, Ni) have been
demonstrated as one of the most promising catalysts, in which Cu
species assists in non-dissociative activation of CO for the CO
insertion and subsequently alcohol formation while FT active
(
2
3
3
4
4
5
0
5
0
5
Limited crude oil reserves and competition with the food industry
associated with fermentation-based biofuel production inspire
new efforts on effective catalytic transformations of alternative
carbon sources to produce energy carriers and chemical
feedstocks. Production of higher alcohols from the catalytic
conversion of synthesis gas (CO + H
gas, or renewable biomass is one of the most promising
approaches for utilizing nonoil resources cleanly and
efficiently. Although Rh-based catalysts are effective for the
formation of ethanol and other C2+ oxygenates from syngas, the
5
5
6
6
7
0
5
0
5
0
elements act as active sites to dissociate CO to form surface
1
1a,2c,3,6
alkyl.
A synergistic effect between these two metal
2
) derived from coal, natural
1a,3,7
compositions is believed to be essential in this reaction.
However, a phase separation of these bimetallic catalysts
generally occurs during the reaction process, which would break
the synergetic interaction and deteriorate the catalytic
1
,2
8
performance. Moreover, the weak thermal conductivity of these
3
very high cost of Rh prohibits its large scale utilization. Recent
powdered catalysts will induce hotspots and cause catalytic
research interest has been focused on the employment of
nonprecious metal catalysts to produce higher alcohols from
syngas over transition metal catalysts (e.g., Cu-Co, Cu-Fe
systems), but improvements in the overall catalytic activity,
alcohol selectivity, and long-term stability of these materials are
highly necessary.
Bimetallic catalysts, composed of two metal elements in either
alloys or intermetallic compounds, emerge as a new materials
category with unique catalytic properties different from
monometallic catalysts through modification of electronic and/or
9
deactivation. Therefore, the design and preparation of new
bimetallic catalysts with desirable activity, selectivity and high
stability toward the production of higher alcohols from syngas
remain a challenging goal.
Layered double hydroxides (LDHs) are a class of naturally
occurring and synthetic materials generally expressed by the
4
2+
3+
n
10
formula [M 1-x
material is that divalent and trivalent cations are uniformly
distributed on the atomic scale in slabs of edge-sharing MO
x 2 2
M (OH) ](A )x/n·mH O. The specialty of this
6
11
octahedra that allow a close interaction of metal cations.
Recently, considerable interest has been focused on LDHs
5
structural factors. For the production of higher alcohols from
This journal is © The Royal Society of Chemistry [2012]
[Green Chem.], [year], [vol], 00–00 | 1

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DOI: 10.1039/C4GC01633E
Green Chemistry
Page 2 of 10
materials as heterogeneous catalysts based on their versatility in
chemical composition and structural architecture. In particular,
a topotactic transformation of LDHs materials to uniformly-
2. Experimental Section
1
2
2
.1 Catalyst preparation
dispersed metal NPs supported on metal-oxide support occurs 45 Preparation of the structured CuCoAl-LDHs, CuAl-LDHs,
1
3
5
0
5
0
5
0
5
upon calcination in a reductive atmosphere. Inspired by the
structural merits of LDHs materials, we explored the idea of the
incorporation of Cu element and FT reaction active Co element
into the LDHs precursor on the atomic scale, so as to fabricate
and CoAl-LDHs film:
The CuCoAl-LDHs, CuAl-LDHs, and CoAl-LDHs film as
catalyst precursors were prepared by an in situ crystallization on
1
4
an aluminum substrate.
The Al substrate was cleaned
supported CuCo bimetallic catalysts toward CO hydrogenation 50 thoroughly with ethanol and deionized water in sequence.
1
1
2
2
3
3
via thetopotactictransformation process.
In a typical procedure, 0.01 mol of Cu(NO
Co(NO ) ·6H
3 2 2
3
)
2
2+
·3H
2
O and
O with a given molar ratio of Cu /Co (5/1, 2/1,
1/2, 1/5, respectively) and 0.06 mol of NH NO were dissolved in
deionized H O (100 mL) to obtain a clear solution, which was
via a facile two-step procedure: an in situ growth of CuCoAl- 55 adjusted to pH=6.5 by adding diluted ammonia (1.5% NH OH).
The Al substrate (15 cm  20 cm) was placed vertically in the
2
+
In this work, core−shell structure Cu@(CuCo-alloy)
nanoparticles (NPs) embedded on the Al
2
O
3
matrix with a high
) were fabricated
4
3
dispersion (denoted as Cu@(CuCo-alloy)/Al
2
O
3
2
4
LDHs nanoplatelets on aluminum substrates as the precursor
followed by a calcination-reduction process (Scheme 1), which
serve as efficient catalysts toward the CO hydrogenation to
produce higher alcohols. The HRTEM and HAADF-STEM
solution in an autoclave, which was placed in a conventional
oven at 80 °C for 48 h. The substrate was then withdrawn from
the autoclave, rinsed with ethanol, and dried at room temperature
results confirm that the well-dispersed core−shell structure 60 (denoted as CuCoAl-LDHs). The other CuAl-LDHs and CoAl-
Cu@(CuCo-alloy) NPs with diameter of 15 nm were embedded LDHs samples were prepared by a similar method with 0.01 mol
on the Al matrix. The resulting materials demonstrate
of Cu(NO ·3H O and Co(NO ·6H O, respectively.
Preparation of the structured Cu@(CuCo-alloy)/Al
Cu/Al , and Co/Al catalyst:
O
3
3
)
2
2
3
)
2
2
2
significantly improved catalytic CO hydrogenation to higher
alcohols, and the best catalytic behavior was obtained over the
2 3
O ,
2
O
3
2 3
O
Cu/Co (1/2) catalyst with a CO conversion of 21.5% and the C6+ 65 The CuCoAl-LDHs, CuAl-LDHs, and CoAl-LDHs precursor
slate 1-alcohols selectivity of 48.9%, which is superior to the
traditional modified FT catalysts. The unique electronic and
geometric interaction between Cu and Co in the Cu@(CuCo-
alloy) NPs give contribution to the significantly enhanced
films were calcined in air at 500 C for 5 h with a heating rate of
–
1
2 C min to obtain the mixed metal oxides (denoted as CuCoAl-
MMO, CuAl-MMO, and CoAl-MMO). Subsequently, the three
samples were reduced in a hydrogen atmosphere at 500 C for 5 h
–
1
catalytic performance, and the bimetal phase separation is 70 with a heating rate of 2 C min . The final catalysts were labeled
inhibited. In addition, the 3D hierachical Cu@(CuCo-
as Cu@(CuCo-alloy)/Al , Cu/Al , and Co/Al
alloy)/Al catalyst facilitates the mass diffusion and
Preparation of the powdered CuCo/Al catalyst:
The powdered CuCo/Al as a reference catalyst was prepared
by a conventional impregnation method, in which the metal
2
O
3
2
O
3
2 3
O .
O
3
O
2 3
2
transportation, and the substrate prevents the hotspot formation
2 3
O
due to its high thermal conductivity. Therefore, our approach
holds significant promise for bimetallic CuCo core–shell 75 contents of Cu and Co were controlled to be 20wt.% and 42wt.%,
structure as a new efficient catalyst toward the production of
higher alcohols from syngas.
respectively (see details in the SI), in accordance with those of
the optimal catalyst Cu@(CuCo-alloy)/Al (Cu/Co=1/2). In a
typical procedure, commercial γ-Al was used as support.
Aqueous solution of Cu(NO ·3H O and Co(NO ·6H O was
2
O
3
2
O
3
3
)
2
2
3
)
2
2
8
0
added dropwise into alumina support with continuous stirring,
followed by aging at room temperature for 4 h. After the
impregnation, the sample was calcined in air at 100 C for 2 h
–
1
and then at 500 C for 5 h (heating rate: 2 C min ). After
hydrogen reduction at 500 C for 5 h with a heating rate of 2 C
–
1
8
5
min , thefinal catalyst was labeled as powdered-CuCo/Al
2 3
O .
2
.2 Characterization of samples
The X-Ray diffraction (XRD) patterns of samples were obtained
on a Shimadzu XRD-6000 diffractometer, using Cu Kα radiation
1
(
λ = 0.154 nm) at 40 kV, 30 mA, a scanning rate of 5° min , a
1
9
0
step size of 0.02° s , and a 2θ angle ranging from 3 to 70°.
Elemental analysis for Cu and Co was performed using a
Shimadzu ICPS-75000 inductively coupled plasma emission
spectrometer (ICP-ES). The sample morphology was investigated
using a scanning electronmicroscopy (SEM;Zeiss Supra 55) with
an accelerating voltage of 20 kV, combined with energy
dispersive X-ray spectroscopy (EDX) for the determination of
metal composition. Transmission electron microscopy (TEM)
was performed using a Hitachi H-800 transmission electron
microscopy operated at 100 kV. High resolution transmission
2 3
Scheme 1. Illustration of the structured Cu@(CuCo-alloy)/Al O catalyst
with core-shell architecture derivedfrom CuCoAl-LDHs film basedon an
in situ growth reactionfollowedby a calcination-reductionprocess.
4
0
9
5

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Page 3 of 10
Green Chemistry
electronmicroscopy (HRTEM) was carried on a JEM -3010 at an
3. Result and discussion
accelerating voltage of 200 kV. Surface elemental analysis was
performed using an ESCALAB250 X-ray photoelectron
spectroscopy (XPS) equipped with Mg Kα radiation. C1s peak at
3
.1 Structural and morphological study of the catalysts
The Fig. 1A illustrates the XRD patterns of the CuCoAl-LDHs
5
0
5
0
5
0
284.6 eV was used as a calibration peak. The Modified Auger 60 film (Cu/Co=1/2) obtained by the in situ growth method on Al
parameter (α') is defined as:
α' = E
are the binding and kinetic energies of the
dominant core electron and Auger electron line for a particular
substrate as well as the corresponding powdered sample scraped
from the substrate for comparison. For the CuCoAl-LDHs film
(curve a), a weak reflection was observed at 2θ 35.1°, which can
b
+ E
k
(1)
b k
where E and E
be attributed to the [012] reflection of the LDHs phase; the strong
1
5
13,16
1
1
2
2
3
element, respectively.
65 reflection appears originating from the Al substrate.
The XRD
Temperature-programmed reduction (TPR) was conducted on a
Micrometric ChemiSorb 2750 chemisorption instrument with a
thermal conductivity detector (TCD). About 100 mg of samples
was loaded in a quartz reactor. TPR was carried out with a
pattern of the powdered material scraped from the LDHs film
(curve b) shows a series of reflections at 2θ 11.7°, 23.5°, 35.1°,
61.0° and 62.5°, corresponding to the [003], [006], [012], [110]
and [113] reflection of an LDHs phase, respectively, which
–
1
heating ramp rate of 5 C min in a stream of 10% H in Ar to a 70 demonstrates the formation of CuCoAl-LDHs film on the Al
2
–
1
11
sample temperature800 C, with a totalflow rate of 25 mL min .
In situ Fourier-transformed infrared absorption (FT-IR)
spectroscopy of CO experiment was carried out in a quartz cell
equipped with KBr windows allowing sample activation and
substrate. The morphology of the LDHs film revealed by SEM
is shown in Fig. 1B and 1C. Top-view and side-view of the LDHs
film show uniform hexagonal plate-like microcrystals with
diameter of 4 µm and thickness of 10–20 nm, whose ab-plane is
successive measurements in the range of 25–600 C, at a pressure 75 perpendicular to the substrate. This is consistent with the XRD
–
4
as low as 10 . About 50 mg of the sample was pressed into a
disk and activated in the same cell used for the measurement. The
thermal treatment was performed either in dynamic vacuum or
under static conditions, according to procedures discussed below.
results in Fig. 1A. Calcination of the LDHs precursor leads to its
transformation to mixed metal oxides (CuCoAl-MMO) (Fig. 1D,
curve a), in which a CuO crystalline phase (JCPDS 89-5899) and
a Co
3
O
4
phase (JCPDS 78-1970) are identified in the XRD
catalyst
2 3
FT-IR spectra were collected with Nicolet 380 instrument 80 pattern. The final structured Cu@(CuCo-alloy)/Al O
spectrophotometer at
accumulation of 64 sans. After nitrogen pre-treatment at 200 C
for 60 min and hydrogenation treatment at 500 C for 60 min, the
sample was scanned to get a background record below a pressure
of 210 Pa. Subsequently, the sample was exposed to a CO 85 indicates the formation of CuCo alloy to some extent during the
flow at 30 C for another 120 min. Sample scanning for adsorbed
–
1
a
spectra resolution of 4 cm and
was subsequently obtained via a reduction process of CuCoAl-
MMO (see Scheme 1). As shown in Fig. 1D (curve b), a broad
reflection at 2θ 44.1° is observed, which can be attributed to the
superimposition of the Cu(111) and Co(111) reflection. This
–
4
reduction process. According to the phase diagram of binary
CO on the studied sample was conducted after the pressure was
Co−Cu, only a maximum of 9at.% Cu can be dissolved in Co
metal. Therefore, the metal Cu, Co and CuCo alloy may coexist
in the final structured sample, which will be further discussed in
the next section.
–
4
17
reduced below 210 Pa again.
.3 Catalyticevaluations
2
9
0
3
4
4
5
5
5
0
5
0
5
Carbon monoxide hydrogenation reaction was carried out in a
fixed bed stainless steel tubular microreactor (8 mm in diameter,
5
00 mm in length). The structured catalyst was rolled and placed
vertically in the stainless steel tubular microreactor. The total
Cu@(CuCo-alloy)/Al O catalyst on the Al substrate was 1.2 g.
2
3
The temperature of the reactor was controlled via a temperature
controller. H , CO and N were purged into the reactor at a
2
2
desired rate by mass flow controllers. Nitrogen was used as an
internal standard gas in the reactor feed. Prior to reaction, the
catalyst was reduced in situ in a flow of H (40 ml/min) under
2
atmospheric pressure at 500 C for 5 h. The reactor was cooled
down to 220 C and synthesis gas with a flow rate of 40 mL/min
(
H
2
: CO=2.0, v/v) was introduced to increase the pressure to 2.0
MPa. During the process, the total pressure in the system was
kept at 2.0 MPa (H /CO=2.0, v/v) with the space velocity of 2000
2
ml/gcat/h. The outlet gas components (CO, H , CH , CO and N )
2
2
4
2
Fig. 1 (A) XRD patterns of: (a) the as-prepared CuCoAl-LDHs
were determined using an online GC-2014C Shimadzu gas
chromatograph by TCD detector (TDX-1 column). The liquid
alcohol and hydrocarbons products were captured using an
ice-water bath and analyzed off-line with the same
chromatograph (a PEG-20 M capillary column and a FID detector
Porapak Q column).
(
Cu/Co=1/2) film, (b) the corresponding powdered material scraped from
the film. (B and C) SEM images of the as-prepared CuCoAl-LDHs film
(the film thickness is shown in the inset of B). (D) XRD patterns of: (a)
CuCoAl-MMO, (b) the final reduction sample. Crystalline phase: (●)
9
5
3 4
Co O , (▼) CuO, (♦) CuCo.

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DOI: 10.1039/C4GC01633E
Green Chemistry
Page 4 of 10
2 17
m ). It has been discussed above that for Cu−Co bimetallic
system, only a maximum of 9 at.% Cu can be dissolved in Co
metal to form CuCo alloy. Therefore, the formation of core–shell
structure (Cu as the core and CuCo alloy as the shell) would
facilitate a maximum CuCo alloy. Moreover, this specific
structure improves the unique electronic and geometric
interaction between Cu and Co species and effectively avoids
phase separation in the catalytic reaction, which will be discussed
in the next section. In contrast, no obvious core−shell structure is
5
5
6
0
5
0
2 3
observed for the powdered-CuCo/Al O sample (Fig. S1B and
S1C). Separate Cu, CoO and Co phase are observed, implying
3
O
4
a phase separation occurs during the reduction process. The
results indicate that a uniform distribution of metal elements in
the LDHs precursor is necessary for the growth of Cu
nanoparticle core and the subsequent CuCo-alloy shell in the
Fig. 2 (A and B) SEM images of the Cu@(CuCo-alloy)/Al
2
O
3
sample
Cu@(CuCo-alloy)/Al
2
O
3
catalyst. This is successfully
(
Cu/Co=1/2); the inset in A shows the photograph of the rolled
demonstrated in the LDHs precursor approach while not available
in the conventional impregnation method.
Cu@(CuCo-alloy)/Al film catalyst. (C) TEM image of the
Cu@(CuCo-alloy)/Al O sample. (D) HAADF-STEM image of the
2
O
3
5
2 3
2 3
Cu@(CuCo-alloy) NPs dispersedonthe Al O matrix with the Cu and Co
EDS mapping(inset).
The architectural feature of the final reduction sample was
revealed by SEM (Fig. 2A), in which the sample inherits the
original flake morphology of the LDHs precursor, and no
agglomeration or sintering of adjacent nanoflakes was observed.
Notably, numerous well-dispersed NPs with a rather high density
1
1
2
2
3
3
4
4
0
5
0
5
0
5
0
5
2 3
on the Al O matrix are observed in Fig. 2B and 2C. The high-
angle annular dark field microscopy (HAADF-STEM) and the
corresponding energy-dispersive spectroscopy (EDS) mapping
demonstrate that both Cu and Co element have a uniform and
homogeneous distribution (Fig. 2D). The detailed structural
feature of the obtained well-dispersed NPs was further revealed
by HRTEM and HAADF-STEM. The HRTEM image (Fig. 3B)
of a nanoflake reveals that the NPs (1520 nm in diamter)
possess a core–shell structure, in which a round core and an
uneven shell (thickness: 2.43.5 nm) can be recognized.
According to the lattice distance of 0.209 nm, the core is
6
7
5
0
2 3
Fig. 3 Structural features of Cu@(CuCo-alloy)/Al O (Cu/Co=1/2)
sample: (A) a low magnification TEM; (B) a high magnification TEM; (C)
HRTEM of a single Cu@(CuCo-alloy) NP; (D) HAADF-STEM for a
single Cu@(CuCo-alloy) NP with the EDS line scan profile along the
pink line: the black and red are the EDS line spectra of Cu–K and Co–K,
respectively.
1
8
determined to be metal Cu (Fig. 3C). However, the shell is not
uniform; several interconnected parts with different lattice
spacings (e.g., 0.206, 0.252, or 0.302 nm) are identified, which
can be assigned to the CuCo alloy phase and some tiny CoO
1
9
phase. This suggests that in the bimetallic CuCo sample, metal
Cu exists in the core section while a CuCo alloy phase
accompanied with tiny CoO phase are located in the exterior shell.
To get a further insight into the structure of the core-shell NPs,
EDS analysis was applied to characterize the structure and
composition of a typical NP. The analysis depth of EDS is
3
.2 The evaluation of catalytic performance
The catalytic performance of the structured Cu@(CuCo-
alloy)/Al (Cu/Co=1/2) catalyst was studied in comparison
with that of the powdered-CuCo/Al (Cu/Co=1/2) one. As
2
O
3
2
O
3
7
8
8
5
0
5
shown in Fig. 4, both catalysts are active in 1-alcohols synthesis,
and the total 1-alcohols selectivity (SROH) increases upon
decreasing the reaction temperature. A SROH value of 50.6% is
0
.53.0 μm, which can detect the whole Cu@(CuCo-alloy)
nanoparticle. As shown in Fig. 3D, the Cu signal is mainly
detected in the central zone; while both Cu and Co signal are
observed in the shell (Fig. 3D, inset). The results provide a
striking demonstration of the core−shell structure: the metal Cu in
the core and both of the two elements in the Co-dominated outer
shell. This suggests that CuO is firstly reduced to metallic Cu
nanoparticle during the reduction process of CuCoAl-MMO (see
Fig. 8), which serves as the core for the further reduction of
obtained for Cu@(CuCo-alloy)/Al
the maximum SROH value is only 33.1% for the powdered-
CuCo/Al catalyst. In addition, the C6+ slate 1-alcohols
selectivity in the total 1-alcohols distribution for Cu@(CuCo-
alloy)/Al at 220 °C reaches to approximately 48.9%, which is
a higher than that of the powdered-CuCo/Al
38.7%). Moreover, it should be noted that CO
2 3
O catalyst at 220 °C, while
2
O
3
2
O
3
2
O
3
(Cu/Co=1/2)
(
2
production is
unwanted but frequently reported to be difficult to exclude
3 4
Co O to metallic Co. Meanwhile, partial Cu atoms migrate to the
2
0
according. As the CO conversion rate increases, an increase of
CO2 production is frequently related to the occurrence of the
surface to form CuCo-alloy shell, due to the relatively lower
surface energy of Cu compared with Co (1.9 J m2 vs. 2.7 J

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Page 5 of 10
Green Chemistry
Table 1 Catalyticperformances of Cu@(CuCo-alloy)/Al
towardthe synthesis of higher alcohols
2
O
3
catalysts with various Cu/Co ratios and thepowdered-CuCo/Al
2
O
3
(Cu/Co=1/2) catalyst
a
b
c
CO
Carbon selectivity (C mol%)
Alcohol Selectivity (wt.%)
Catalyst
Co/Al
conv. (%)
CO
2
ROH
HC
MeOH
2.3
EtOH
C
3-5OH
C6+OH
2
O
3
36.6
32.9
21.5
20.7
21.9
22.3
2.3
4.7
7.2
7.9
8.6
9.5
2.6
95.1
59.1
42.2
39.6
24.6
3.8
2.6
27.1
18.5
15.4
22.3
9.2
68.0
49.6
48.9
39.2
13.8
0.2
Cu/Co (1/5)
Cu/Co (1/2)
Cu/Co (2/1)
Cu/Co (5/1)
36.2
50.6
52.5
66.8
86.7
15.1
19.2
20.9
66.7
98.9
16.8
16.5
17.6
10.3
0.6
Cu/Al
powdered-
CuCo/Al (1/2)
2
O
3
0.3
1
3.1
9.8
22.9
67.3
22.6
19.8
18.9
38.7
2
O
3
a
Reaction conditions: Temp.= 220 C, P=2MPa, R(H
2
/CO)=2, GHSV=2000 ml/gcat/h.
b
5
Carbon selectivity is defined as the selectivity of all the carbon-containing products from converted carbon, and the values are recalculated from the
original data; HC = total hydrocarbons includingmethane; ROH = total alcohol includingmethanol.
c
Alcohol distribution(wt.%): the proportionof eachalcohol in the total value.
the performances of these bimetal samples is observed,
. In this work, for 50 suggesting strong coppercobalt interaction which is
2 3
O , the increase of the CO conversion from responsible for thecatalytic selectivity.
water-gas shift reaction, CO + H
Cu@(CuCo-alloy)/Al
00 to 260 °C does not entail drastic change in the CO
2
O → CO
2
+H
2
a
1
1
2
2
3
3
4
4
0
2
2
production; while predominant CO
2
is produced over powdered-
CuCo/Al O (Cu/Co=1/2) catalyst in the same temperature range.
2
3
The most intriguing observation in Fig. 4 is the high α-chain-
lengthening probability of 1-alcohol formation for the
Cu@(CuCo-alloy)/Al O catalyst. The α-value ranges from 0.79
5
0
5
0
5
0
5
2
3
to 0.62 at the reaction temperature between 200 and 280 °C ,
which therefore achieves the purpose of maximizing the yields of
6+ slate 1-alcohols. However, the powdered-CuCo/Al O
2 3
(Cu/Co=1/2) catalyst fails to produce C6+ slate 1-alcohols (Fig. 4
and Table 1), with the α-value ranging in 0.680.40 at the same
reaction temperature. The results indicate that the Cu@(CuCo-
alloy)/Al O with unique core–shell structure derived from the
2 3
LDHs precursor exhibits significantly enhanced catalytic
behavior.
C
It is well-known that Cu is the major element for methanol
synthesis, serving as the dissociative chemisorption of hydrogen
and the associative adsorption of CO; while Co affords the active
site of FT function of dissociative CO adsorption (CC chain
1
a,3,7
growth) and hydrogenation.
Accordingly, the synergetic
effect of Cu and Co plays a key role in determining the catalytic
performance. Therefore, a detailed catalytic performance study of
this binary system was performed by changing the relative
amount of Cu and Co. As shown in Table 1, the selectivity
Fig. 4 Catalytic performances of Cu@(CuCo-alloy)/Al
and powdered-CuCo/Al (Cu/Co=1/2) catalyst, respectively.
catalysts change 55 Temp.=220 C, P=2 MPa, GHSV=2000 ml/gcat/h, H
/CO=2.0. The α-
catalyst exhibits the
chain-growth probability is calculated according to
ln(W /n)=nlna+ln(1α) /α, in which n is the number of carbon atoms in 1-
2 3
O (Cu/Co=1/2)
2
O
3
patterns of various Cu@(CuCo-alloy)/Al
accordingly. The monometallic Co/Al
2
O
3
2
2
O
3
2
n
highest activity in Fischer–Tropsch synthesis producing mostly
alcohol and W is the weight fraction of 1-alcohol that contains n carbon
5.1% of hydrocarbons with only trace of alcohols (2.6%); while
n
9
atoms.
methanol (98.9%) is major reaction product with the lowest
selectivity of hydrocarbons for monometallic Cu/Al catalyst. 60 Time-on-stream analysis of the best-performing Cu@(CuCo-
2 3
O
All the CuCo bimetallic catalysts are selective between alcohols
and hydrocarbons at 220 °C , i.e., with the increase of Cu/Co ratio,
the alcohols selectivity increases while the hydrocarbons
selectivity decreases gradually. However, the selectivity of C6+
alloy)/Al (Cu/Co=1/2) catalyst was investigated at 220 C
2 3
O
with the relative pressure of 2 MPa for up to 48 h as shown in Fig.
5. The catalyst selectivity decreases to a slight extent during the
initial 10 h and then maintains at a constant value (43%). This
slate 1-alcohols (SC6+OH) shows a primary enhancement followed 65 stable activity can be ascribed to the specific core–shell structure
by a sharp decline, and the highest SC6+OH value of 48.9%
presents in the sample of Cu/Co (1/2), with the SROH value of
of Cu@(CuCo-alloy) NPs in which an unique electronic and
geometric interaction between Cu and Co species suppresses the
5
0.6% and SCO2 value of 7.2%. To sum up, a distinct difference in
2 3
phase separation; the grafting of active NPs into the Al O matrix

View Article Online
DOI: 10.1039/C4GC01633E
Green Chemistry
Page 6 of 10
guarantees
hierarchical Cu@(CuCo-alloy)/Al
tunnels, facilitating the mass diffusion and transportation.
HRTEM images of the used Cu@(CuCo-alloy)/Al catalyst are
shown in Fig. S2, in which no obvious aggregation is observed 50 size and shell thickness can be obtained by controlling the Cu/Co
a
satisfactory stability. Furthermore, the 3D
Cu@(CuCo-alloy) NPs was calculated to be 9.2, 13.9, and 18.1
nm for Cu/Co (2/1), Cu/Co (1/2), Cu/Co (1/5), with the thickness
of 1.8, 4.1, and 6.2 nm, respectively. In brief, the core–shell
2
O
3
catalyst provides open
2
O
3
2 3
structure Cu@(CuCo-alloy)/Al O catalysts with various particle
5
and the bimetallic CuCo NPs maintain the core–shell structure
with a mean size of 30 nm. In the case of the used powdered-
CuCo/Al O (Cu/Co=1/2) catalyst however, an increase in the
2 3
mean size (42 nm) of bimetallic CuCo NPs was observed (Fig.
S3).
ratio in CuCoAl-LDHs precursors. The alloy degree and shell
thickness will influence the surface composition of catalytic
active sites, which will be further studied by the following
investigations.
1
0
5
5
Fig. 5 Product selectivities as a function of time on stream over the
Fig. 6 XRD patterns of Cu@(CuCo-alloy)/Al
Cu/Co ratios: (a) Cu/Co=5/1, (b) Cu/Co=2/1, (c) Cu/Co=1/2, (d)
Cu/Co=1/5. Crystalline phase: (●) Cu, (♦) Co, (▼) CuO, (■) Co
2 3
O catalysts with various
2 3
Cu@(CuCo-alloy)/Al O catalyst (Co /Cu=1/2) performed at 220 °C, 2
MPa andH /CO=2.
2
O .
3 4
1
5
3.3 Correlation studies on the structure and catalytic
behavior
The results above clearly show that the Cu@(CuCo-
alloy)/Al O catalysts with core−shell structure display excellent
2 3
catalytic behavior. To give an understanding of structure–
performance relationship, XRD, TPR, XPS and FTIR
measurements were performed to elucidate the electron and
geometry structure for these bimetal catalysts. The nominal and
determined metal ratios of the products by inductively coupled
plasma emission spectroscopy (ICP-AES) are also summarized in
Table S1. Fig. 6 illustrates the XRD patterns of the Cu@(CuCo-
2
2
3
3
4
4
0
5
0
5
0
5
alloy)/Al
2
O
3
catalysts with various Cu/Co ratios. For the Cu/Co
(5/1) sample, the Cu(111) and Co(111) reflection are observed at
2
θ 43.3° and 44.3°, respectively, suggesting the separate Cu and
Co phase without the formation of a significant degree of solid
solution. With the decrease of Cu/Co ratio, the reflections of Cu
and Co metal gradually combine together. The Cu/Co (1/5)
sample exhibits a single reflection located at between metallic Cu
6
0
Fig. 7 HRTEM images of the Cu/Al
samples with various Cu/Co ratios: (A) Cu/Al
Cu/Co=1/2, (D) Cu/Co=1/5.
2
O
3
and Cu@(CuCo-alloy)/Al
2 3
O
2
O
3
, (B) Cu/Co=2/1, (C)
(
2 [111] = 43.3°) and Co (2 [111] = 44.3°), which implies that
The H
reducibility and synergistic effect of Cu−Co in Cu@(CuCo-
alloy)/Al samples and the corresponding profiles are shown in
2
-TPR measurements were conducted to investigate the
Cu atoms are completely incorporated into the Co lattice to the
formation of CuCo alloy phase. This result indicates that the
degree of alloy can be tuned by changing the Cu/Co ratio in the
CuCoAl-LDHs precursor, and a lower Cu/Co ratio facilitates the
formation of CuCo alloy. The XRD pattern of used Cu@(CuCo-
6
5
2 3
O
Fig. 8. The sample of CuAl-MMO (Fig. 8, curve a) exhibits a
main peak at 193 C, which is attributed to the reduction of CuO
2
1
to metal Cu; while the sample of CoAl-MMO sample (Figure 8,
alloy)/Al
phase is stable after the reaction (Fig. S4). As shown in Fig. 7, the
Cu NPs in the Cu/Al sample show a particle size of 13.2 nm
Fig. 7A). With the decrease of Cu/Co ratio in the bimetal
2 3
O (Co/Cu=1/2) catalyst shows that the CuCo alloy
curve e) displays two peaks at 371 C and 568 C, assigning to
2
2
7
0
the reduction of Co
case of CuCoAl-MMO samples (Fig. 8, curve bd), both the
reduction of CuO (in the low temperature range) and Co (in
3 4 3 4
O in two steps (Co O /CoO/Co). In the
2
O
3
(
3
O
4
catalysts, both the particle size and the shell thickness of
Cu@(CuCo-alloy) NPs increase in the following order: Cu/Co
(2/1)<Cu/Co (1/2)<Cu/Co (1/5) (Fig. 7B–D). The average size of
the high temperature range) are observed. Obviously, compared
with the CoAl-MMO sample, the broad hydrogen consumption

View Article Online
DOI: 10.1039/C4GC01633E
Page 7 of 10
Green Chemistry
peak for the reduction of Co
68 C to 359 C) along with the increase of Cu content (from
Cu/Co=0/1 to Cu/Co=5/1), indicating a Cu-promoted reduction of
3 4
O shifts to lower temperature (from 45 existence of predominant Co metal in the shell section. Notably,
5
the surface Cu/Co ratio is bigger than the bulk ratio for the Cu/Co
(2/1) sample, which is indicative of the migration of copper atom
to the crystallite surface to form a CuCo alloy shell. It is therefore
reasonable to conclude that the reduced bimetal NPs possess a
2
3
Co species to produce CuCo alloy.
However, the H
2
5
0
5
0
5
0
5
consumption peak of CuO phase shifts gradually to higher
temperature (from 193 C to 279 C) along with the increase of 50 core–shell structure, with predominant Cu in the core and CuCo
Co content (from Cu/Co=1/0 to Cu/Co=1/2). The results suggest
alloy in the outer shell. Furthermore, it was worth noting that the
Cu 2p3/2 peak shifts from 931.9 (Cu-only) to 932.4 eV upon
alloying with increased Co (Cu/Co=1/5) (Fig. 9, from curve e to
a); while a shift from 780.9 (Co-only) to 780.0 eV (Cu/Co=2/1)
a strong synergistic effect between copper and cobalt occurs in
2
2,24
the reduction process of CuCoAl-MMO samples.
To further confirm the formation of the core–shell structure
3
and the active site state of the Cu@(CuCo-alloy)/Al O catalyst, 55 can be observed for the Co 2p3/2 peak (Fig. 10, from curve a to d).
1
1
2
2
3
3
2
XPS measurements were performed. For all the sample, two
An upshift of Co 2p3/2 is not observed for Cu/Co (5/1) sample
peaks (Fig. 9A) centered at 931.9 and 951.5 eV are mainly
ascribed to Cu 2p3/2 and Cu 2p1/2 peaks of Cu and/or Cu ,
(Fig. 10, curve e), probably due to a portion of unalloyed Co.
0
+
0
This indicates the diminishing electron density in the Cu nucleus,
0
29
respectively, which is difficult to differentiate these two species
resulting from the electron scavenging property of Co .
2
4
based on their Cu 2p binding energies. Therefore, the Modified 60 Therefore, a strong electron interaction between Cu and Co
0
+
Auger parameter (α') was used to distinguish Cu from Cu
occurs on the catalyst surface, wherein electrons likely transfer
from Cu species to Co species in the bimetal Cu@(CuCo-
alloy)/Al O catalyst.
2 3
species, which is an important energy parameter for identifying
the chemical state of elements where chemical shift is very small
1
5
or comparable with the energy resolution of the instrument. As
shown in Table S2, the Modified Auger parameter for the
Cu@(CuCo-alloy)/Al
close to the reported value for Cu (1851.3 eV). For the used
Cu@(CuCo-alloy)/Al (Co/Cu=1/2) catalyst (Fig. S6), the Cu
LMM XAES spectrum shows an Auger peak of Cu (915.6 eV)
2 3
O samples with various Cu/Co ratios is
0
25
2
O
3
+
0
26
besides that of Cu . However, the Modified Auger parameter is
0

1851.2 eV, indicating only slight Cu specie is oxidized during
2
5
the reaction process. For the Co 2p XPS spectra of fresh and
used Cu@(CuCo-alloy)/Al catalyst (Fig. 10 and Fig. S8), a
broad peak in the range 775785 eV is observed, which is
2
O
3
2
7
difficult to determine the chemical state of Co species. However,
the Modified Auger parameter for these samples ranges in
0
1
(
550.91552.2 eV (Table S2), close to the reported value for Co
6
5
Fig. 9 Cu XPS spectra of Cu/Al
with various Cu/Co ratios: (a) Cu/Co=1/5, (b) Cu/Co=1/2, (c) Cu/Co=2/1,
(d) Cu/Co=5/1, (e) Cu/Al
2 3 2 3
O and Cu@(CuCo-alloy)/Al O samples
2+
28
1551.2 eV) other than Co (1553.8 eV). This indicates that
Co is the predominant species both in the fresh and used
0
2 3
O .
Cu@(CuCo-alloy)/Al catalyst and the structure of CuCo alloy
2
O
3
shell remains stable during the reaction process. As listed in
Table 2,
2 3 2 3
Fig. 10 Co XPS spectra of (a) Co/Al O and Cu@(CuCo-alloy)/Al O
samples with various Cu/Co ratios: (b) Cu/Co=1/5, (c) Cu/Co=1/2, (d)
7
0
Cu/Co=2/1, (e) Cu/Co=5/1.
In situ FTIR is a useful surface-sensitive technique to study the
adsorption behavior of catalysts under reaction conditions and to
elucidate the nature of the active sites and the surface
intermediates involved in the reaction. Fig. 11 displays IR spectra
4
0
2
Fig. 8 H -TPR profiles of CuAl-MMO, CoAl-MMO and CuCoAl-MMO
samples with various Cu/Co ratios: (a) CuAl-MMO, (b) Cu/Co=5/1, (c)
Cu/Co=2/1, (d) Cu/Co=1/2 and(e) CoAl-MMO.
7
5
of CO adsorbed on Co/Al
samples with different Cu/Co ratios (5/1, 2/1, 1/2, 1/5). The IR
spectraof Co/Al , bimetallic Cu/Co (1/5) and Cu/Co (1/2)
2 3 2 3
O and Cu@(CuCo-alloy)/Al O
the Cu/Co ratio at surface (2.88/1.00) is much smaller than the
bulk ratio for the Cu/Co (4.81/1.00) sample, indicating the
2
O
3

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DOI: 10.1039/C4GC01633E
Green Chemistry
Page 8 of 10
Table 2 Cu/Co ratio at the surfaceandin the bulk for the bimetal
2 3
alloy)/Al O (Cu/Co=1/2) catalyst, copper species provides active
site for the associative adsorption of CO to produce CO*; while
Cu@(CuCo-alloy)/Al
2 3
O catalysts
cobalt species acts as active site for CO dissociation, C–C chain
molar Cu/Co ratio
Bulk (ICP)
Norminal ratio
55 growth and hydrogenation to form *C
to the *C group and inserts via surface migration over a short
distance between Co and Cu site and subsequent hydrogenation
n z
H group. The CO* moves
Surface (XPS)
2.88/1.00
2.07/1.00
1.00/2.86
1.00/5.02
n
H
z
Cu/Co (5/1)
Cu/Co (2/1)
Cu/Co (1/2)
Cu/Co (1/5)
4.81/1.00
1.96/1.00
1.00/2.63
1.00/4.76
3
,7
to produce higher alcohols. A homogeneous distribution of
copper and cobalt as well as their distance (geometric properties)
are necessary to obtain higher alcohols. For the electron
interaction between Cu and Co, the electron transfer from Cu to
Co increases the electron density of Co, which weakens the CO
bond of the adsorbed CO and facilitates its dissociation; while the
6
0
catalyst (with predominant Co) exhibit strong bands with
1
5
0
5
0
5
0
5
0
5
0
maximum at 1991, 1980, and 1959 cm , respectively, which can
decreased electron density of Cu enhances the associative
3
0
35
be assigned to the bridge-type adsorbed CO on Co metal sites.
In comparison with the monometallic Co/Al catalyst (1991
cm ), the significant shifting of this peak to lower frequencies
65 adsorption of CO. Therefore, both the geometric effect and
electronic effect between Co and Cu contribute to the
enhancement of selectivity toward higher alcohols.
2
O
3
1
1
(
from 1991 to 1959 cm ) for the bimetallic samples along with
1
1
2
2
3
3
4
4
5
the increase of Cu content indicates the surface Co species is
2
9b
slightly negatively charged.
That is, electron transfer from
surface Cu species to Co species occurs, as also supported by the
XPS data discussed above. Upon further increasing the Cu
content (Cu/Co=2/1 sample: Fig. 11, curve d), the bridge-type
adsorbed CO disappears completely, while the linearly adsorbed
CO on Co metal is observed. The peak fitting of the band
observed in this region reveals the presence of two different

1
0
peaks (2015 and 2038 cm ), which point to different Co sites.
1
The Co-carbonyl band at 2015 cm can be ascribed to
coordinatively unsaturated Co sites such as those located in steps
or corners; while the contribution at 2038 cm can be attributed
to Co sites located in planar (terrace) positions. In addition, the
IR spectra of CO adsorbed on Cu/Co (1/2), Cu/Co (2/1), and
0
1
0
31
2 3
Fig. 11 In situ FTIR spectra of CO adsorption on (a) Co /Al O , and
Cu/Co (5/1) samples (Fig. 11, curve c, d, e) shows an absorption 70 Cu@(CuCo-alloy)/Al O samples with various Cu/Co ratios: (b)
2
3
1
band at 2085, 2095 and 2096 cm , respectively, accompanying
Cu/Co=1/5, (c) Cu/Co=1/2, (d) Cu/Co=2/1, (e)Cu/Co=5/1.
0
with gradually enhanced intensity, which points to the Cu
3
2
surface site. In addition, this peak shifts to higher frequencies
from 2085 to 2096 cm ) along with the increase of Cu content,
4. Conclusions

1
(
2 3
In summary, a structured core−shell Cu@(CuCo-alloy)/Al O
further indicating the electron transfer from surface Cu species to
Co species. It can be seen from Fig. 11 that the CO adsorption
catalyst was fabricated via a facile two-step procedure: the direct
75 growth of CuCoAl-LDHs nanoplatelets on aluminum substrates
followed by a calcination-reduction process, which serves as an
efficient catalyst toward the CO hydrogenation (CO conversion:
21.5%; C6+ slate 1-alcohols selectivity: 48.9%). The specific
core-shell structure Cu@(CuCo-alloy) NPs possess a Cu-rich
0
state on Co species gradually transfers from bridge-type to
linear-type along with the increase of Cu content, which can be
3
3
ascribed to “active-site isolation” effect imposed by Cu and thus
0
0
a close interaction of Cu and Co species.
Previous researchers have concluded that the linearly-adsorbed
CO is the active site for the formation of oxygenated compounds,
while the bridge-type adsorbed CO leads toward formation of
hydrocarbon compounds, since the latter one has a weaker C–O
8
0
core and
a
CuCo-alloy shell, which improves the
electronic/geometric interaction between Cu and Co and
effectively avoids phase separation during the catalytic reaction.
In addition, the open channels associated with the hierarchical
structure facilitate the mass diffusion/transport as well as inhibit
the formation of hotspots. This strategy can be extended to
synthesize other bimetallic catalysts derived from LDHs and
offers new opportunities for achieving largely enhanced catalytic
preformances based on bimetal synergistic effect.
2
9b,34
bond and thus can be more easily hydrogenated.
catalytic evaluation results (Table 1) show that the Co-rich
catalysts (Co/Al , Cu/Co (1/5) with dominated bridge-type
The
8
5
2
O
3
adsorption) are more selective toward hydrocarbons; while the
Cu-rich catalysts (Cu/Co (2/1), Cu/Co (5/1) with dominated
linear-type adsorption) are prone to obtain oxygenated
compounds. According to the reaction pathway for CO
1
a,3,7
hydrogenation over the CuCo-based catalysts (Fig. S9),
Cu is
9
0
Acknowledgements
the major element for methanol synthesis, serving as the
associative adsorption of CO; while Co affords the active site of
FT function of dissociative CO adsorption and hydrogenation. A
synergistic effect between these two metal compositions is
believed to be essential in this reaction. For the Cu@(CuCo-
This work was supported by the 973 Program (Grant no.
2
011CBA00504), the National Natural Science Foundation of
China (NSFC), the Beijing Natural Science Foundation

View Article Online
DOI: 10.1039/C4GC01633E
Page 9 of 10
Green Chemistry
(
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no. JX20140015). M. Wei particularly appreciates the financial
aid from the China National Funds for Distinguished Young
Scientists of the NSFC.
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Green Chemistry
Page 10 of 10
View Article Online
DOI: 10.1039/C4GC01633E
Core−Shell Cu@(CuCoꢀalloy)/Al O Catalysts for the Synthesis of
2
3
Higher Alcohols from Syngas
‡a
‡b
a
a
a
a
Wa Gao, Yufei Zhao, Haoran Chen, Hao Chen, Yinwen Li, Shan He, Yingkui
a
a
a
a
Zhang, Min Wei,* David G. Evans and Xue Duan
a
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical
Technology, Beijing, 100029, China. E-mail: weimin@mail.buct.edu.cn
b
Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing, P. R. China.
These authors contributed equally.
‡
Graphical Abstract
Core−shell Cu@(CuCoꢀalloy)/Al O catalysts were fabricated via a facile twoꢀstep procedure:
2
3
in situ growth of CuCoAlꢀLDH nanoplatelets on aluminum substrates followed by a
calcinationꢀreduction process, which exhibit excellent catalytic behavior toward CO
hydrogenation to produce higher alcohols.