Carbon nanotube-supported copper-cobalt catalyst for the production of higher carbon number alcohols through carbon monoxide hydrogenation

Article abstract of DOI:10.21577/0103-5053.20170237

A series of carbon nanotube (CNTs)-supported copper-cobalt catalysts were prepared and investigated in a slurry reactor for their ability to selectively convert syngas into higher carbon number alcohols. The 7.5Cu7.5Co/CNTs catalyst achieved superior selectivity towards the formation of ethanol (30.1percent) and C₂₊ alcohols (57.7percent), while the 10Co5Cu/CNTs catalyst exhibited the largest alcohol space-time yield (372.9 mg g_cat^-1 h^-1). However, the pure Cu (15Cu/CNTs) catalyst displayed negligible activity. Cobalt reduction was enhanced in the presence of copper. In addition to the Cu⁰-Co⁰ center, Co⁰-Co²⁺ also presented dual active sites for higher alcohols synthesis, the Co²⁺ site could terminate carbon chain growth to produce alcohols. The ratio of Cu/Co considerably influences the metal particle properties-synergistically effecting the active species.

Full text of DOI:10.21577/0103-5053.20170237

http://dx.doi.org/10.21577/0103-5053.20170237
J. Braz. Chem. Soc., Vol. 29, No. 7, 1373-1381, 2018
Printed in Brazil - ©2018 Sociedade Brasileira de Química
Article
Carbon Nanotube-Supported Copper-Cobalt Catalyst for the Production of Higher
Carbon Number Alcohols through Carbon Monoxide Hydrogenation
Peng Wang,a Xihua Du,*,a Wenchang Zhuang,a Keying Cai,a Jing Li,a Yan Xu,a
Yingmei Zhou,a Kai Sun,b Shuyao Chen,b Xiaoli Lib and Yisheng Tan*,b
aSchool of Chemistry and Chemical Engineering, Xuzhou Institute of Technology,
221018 Xuzhou, China
bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry,
Chinese Academy of Sciences, 030001 Taiyuan, China
A series of carbon nanotube (CNTs)-supported copper-cobalt catalysts were prepared and
investigated in a slurry reactor for their ability to selectively convert syngas into higher carbon
number alcohols. The 7.5Cu7.5Co/CNTs catalyst achieved superior selectivity towards the
formation of ethanol (30.1%) and C2+ alcohols (57.7%), while the 10Co5Cu/CNTs catalyst exhibited
-1
the largest alcohol space-time yield (372.9 mg gcat h-1). However, the pure Cu (15Cu/CNTs)
catalyst displayed negligible activity. Cobalt reduction was enhanced in the presence of copper.
In addition to the Cu0–Co0 center, Co0–Co2+ also presented dual active sites for higher alcohols
synthesis, the Co2+ site could terminate carbon chain growth to produce alcohols. The ratio of Cu/Co
considerably influences the metal particle properties-synergistically effecting the active species.
Keywords: CO hydrogenation, higher alcohols synthesis, CuCo, carbon nanotube
Introduction
known that Co species are the center for CO dissociation,
hydrogenation and chain propagation to form surface
alkyl species, while Cu species are the site for molecularly
absorbed CO insertion and alcohol formation.9,10 The
synergistic effect and the surface distribution of the two
active components have significant influence on the
catalytic performance for the synthesis of alcohols.11,12
It has been demonstrated that the porous structure of the
support may efficiently control metal dispersion.13,14 For
supported catalysts, metal particle size is typically limited
by the pore size of the support and the support texture
may prevent sintering of the metal particles15 or limit
sintering to the maximum pore diameter.16,17 Furthermore,
it is expected that the nature of the support may influence
copper-cobalt interactions, which may lead to different
active phase structures and properties. SiO2, Al2O3 and
TiO2 have different interactions with cobalt species.
Strongly interacting supports such as γ-Al2O3 usually
lead to high cobalt oxide dispersions, however, there is
difficulty to reduce cobalt species.18 To obtain appropriate
Cu and Co nanoparticle properties and distribution in
the catalyst, carbon nanotubes (CNTs) are employed as
supports. A wealth of interdependent research19-21 has
demonstrated the advantages of using CNTs as supports
The conversion of synthesis gas derived from coal,
natural gas or biomass into higher carbon number
alcohols is both attractive and challenging to the field of
C1 chemistry.1-3 The alcohols obtained from this process
can be used as fuel blends or as value-added chemicals in
fine chemical synthesis.4,5 Furthermore, higher alcohols
synthesis (HAS) is a prototypical example of reactions
that rely on synergistic effects between dual catalytic sites
with different functionalities, one active site catalysis
CO dissociation, while the second site enables CO
non-dissociation centers to form.6 Typically, among the
available non-precious metal catalysts, modified bimetallic
Fischer-Tropsch (FT) catalysts (e.g., CuCo) are widely
investigated as one of the most promising candidates for
HAS.7,8
Therefore, synergistic combination of two metals
represents an efficient approach towards desirable catalytic
properties by the creation of hybrid sites, and intimate
contact between the metal components has been recognized
as important to achieve satisfactory selectivity. It is
*e-mail: 12dxh@sina.com; tan@sxicc.ac.cn

1374
Carbon Nanotube-Supported Copper-Cobalt Catalyst for the Production of Higher Carbon Number Alcohols
J. Braz. Chem. Soc.
to improve transition metal particle dispersion in catalysts
and the confinement effects during catalysis have been
systematically studied. Previous studies concluded that
space restriction,22 enrichment of reactants inside CNTs23
and the electronic interaction of the confined materials
within CNTs24 is expected to facilitate cobaltous oxide
reduction. Therefore, there is a need to minimize the
distance between dual catalytic sites, while maintaining
active particles of sufficiently small size with a high
degree of dispersion induced by the confinement effects
of nanotubes.
In the present study, a series of CNTs-supported
CuCo-based precursors with different Cu/Co ratios were
synthesized using a simple and effective co-impregnation
method. After calcination, a homogeneous and highly
dispersed bimetallic catalyst was obtained, as revealed by
high-resolution transmission electron microscopy (HRTEM)
analysis. In situ X-ray photoelectron spectroscopy (XPS)
indicated Cu0–Co0 and Co0–Co2+ centers were the active
sites for HAS in our system. The catalytic performance
of the catalysts prepared in this work towards the
hydrogenation of carbon monoxide was examined in a
slurry reactor. The maximum space-time yield (STY)
reached 372.9 mg gcat-1 h-1 and an ethanol selectivity as high
as 30.1% was obtained. Improved synergistic effects were
observed between the dual active sites at a Cu/Co ratio
range of 0.5-2, with the ratio exerting significant influence
on the catalytic performance.
Synthesis of Cu/Co-based catalysts
The CNTs were immersed in an ethanolic solution
of precursor salts of Cu, Co and then subjected to
ultrasonic treatment and stirring. The ultrasonic treatment
and stirring facilitated the filling of the CNT channels
with the precursor solution during impregnation. After
10 h of impregnation, the mixture was dried at 60 °C
overnight, followed by heating to 110 °C for 10 h. This
process afforded a homogeneous distribution of the active
components inside the channels of the CNTs.After drying,
the products were calcined in N2 at 673 K for 4 h with a
heating rate of 2 K min-1.A series of catalysts were prepared
using this method, i.e., 15Co/CNTs, 12.5Co2.5Cu /CNTs,
10Co5Cu/CNTs, 7.5Cu7.5Co/CNTs, 10Cu5Co/CNTs,
12.5Cu2.5Co/CNTs, 15Cu/CNTs, and the active
components supported on CNTs with closed caps was
denoted as 10Cu5Co-out-CNTs. These numbers stand for
the loading metals weight contents.
Characterization
Powder X-ray diffraction patterns (XRD) were recorded
over the 2θ range from 5 to 85° using a Rigaku MiniFlex II
X-ray diffractometer, which was operated at 40 kV and
40 mA with Cu Kα radiation (k = 0.15418 nm) .
H2-temperature-programmed reduction mass
spectrometry analysis (H2-TPR-MS) was performed
to investigate the reducibility of the catalysts using a
chemisorption instrument (TP-5080) and the sweep gas was
detected using an OmniStar instrument. The catalyst (50 mg)
was pre-treated at 300 °C under a flow of N2 (32 mL min-1)
for 1 h and then cooled to 50 °C; after cooling, the flow
was changed to a H2/N2 = 0.09 mixture (35 mL min-1). The
temperature-programmed reduction was performed between
50 and 800 °C with a heating rate of 10 °C min-1.
X-ray photoelectron spectroscopy (XPS) and X-ray
Auger electron spectroscopy (XAES) were used to
analyze the change in the surface composition using an
AXIS ULTRA DLD instrument equipped with Al Kα
(hv = 1486.6 eV). The sample was reduced in situ under a
flow of H2 at 400 °C for 1 h, and the following photoelectron
lines were recorded: Cu 2p, Co 2p, C 1s and O 1s. The
binding energy values were corrected for charging effects
by referring to the adventitious C1s line at 284.5 eV.
The chemical compositions (Cu and Co) of the
as-prepared samples were determined by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES)
using a Thermo iCAP 6300 instrument.
Experimental
Materials
Analytical-grade chemicals, including Cu(NO3)2⋅3H2O
and Co(NO3)2⋅6H2O were purchased from the Beijing
Chemical Co., Ltd. and were used as received without
further purification. Multi-walled CNTs (outer
diameter < 8 nm, lengths 10-30 µm) were purchased
from Chengdu Organic Chemicals. High-resolution
transmission electron microscope (HRTEM) analysis
showed that the average inner diameter of CNTs was
around 3.9 nm and the wall thickness of it was in the
range of 1.5-3 nm. Raw CNTs were refluxed in HNO3
(68 wt.%) for 14 h at 140 °C in an oil bath to purify and
cut the carbon tubes, in contrast, CNTs with closed caps
were obtained by refluxing CNTs in 37 wt.% HNO3 for
5 h at 100 °C. Then, the mixture was filtered and washed
with deionized water, followed by drying at 60 °C for 12 h.
HRTEM analysis indicated that the ends of the CNTs were
open and the lengths of them were 0.5-2 µm.
The textural properties of catalysts were measured by
using the N2-sorption isotherm at 77 K on a Micromeritics

Vol. 29, No. 7, 2018
Wang et al.
1375
ASAP2020 analyzer. Prior to each measurement, the
sample was degassed at 200 °C overnight. The specific
surface area was calculated by using the Brunauer-Emmett-
Teller (BET) equation. The micropore volume was obtained
from the t-plot method. The mesopore volume was obtained
from the Barrett-Joyner-Halenda (BJH) method using
the desorption branch. The pore size distributions were
evaluated by using the density functional theory (DFT)
method applied to the nitrogen adsorption data.
The morphologies and microstructures of the materials
were observed using a high-resolution transmission electron
microscope (JEOL, JEM-2100F), which was operated at
200 kV, with a lattice resolution of 0.14 nm.
was observed at a 2θ value of 25.8° and the intensity
of peaks decreased gradually as a function of increased
cobalt loading. The 15Cu/CNTs catalyst primarily
consisted of a Cu2O phase, with corresponding 2θ peak
reflections positioned at 36.5, 42.4, 61.5, 73.7 and 77.6°
(JCPDS No. 65-3288). Additionally, the CuO phase was
also present in both the 15Cu/CNTs and 12.5Cu2.5Co/CNTs
samples. As observed, the diffraction peak intensity of
the CuO and Cu2O species decreased gradually as the
loading of Cu decreased. Furthermore, when the Co
loading exceeded that of Cu, peaks corresponding to CoO
(JCPDS No. 65-5474) were observed at very low intensities
in these samples. In addition, carbon nanotubes seem to
stabilize the CoO phase, which is usually unstable under
ambient conditions. No other XRD characteristic peaks and
significant shift of peaks were observed in these patterns,
as bulk CoCu alloy may not be formed in the catalysts,
due to the low solubility between Co and Cu. Furthermore,
CuCo2O4 spinel or Cu2+ Co2+1-xCo3+ O4 structure may also
Catalysis studies
Carbon monoxide hydrogenation reactions were
performed in a 0.5 L continuous slurry reactor equipped
with a mechanical agitator. The reactions were conducted
at a temperature of 573 K and pressure of 4.5 MPa, using a
feed gas of a 2:1 mixture of H2 and CO at a space velocity
x
2
not exist in these samples because of the absence of Co3O4
phase.25
-1
of 10000 mL gcat h-1. The catalysts were reduced with a
10% hydrogen-in-nitrogen mixture for 3 h at 673 K in a tube
furnace. After cooling to room temperature under a flow of
high purity nitrogen, the reduced catalysts were added into an
agatemortarwithliquidparaffin, thesolidparticleswerefinely
powdered, then the mixed liquid paraffin were transferred
into the slurry reactor. All experimental data were obtained
under steady-state conditions that were invariably maintained
for 7 h and no deactivation was observed. The products
were analyzed using five online gas chromatographs (GCs)
during the reaction. The organic gas products, consisting of
hydrocarbons and methanol, were detected online by flame
ionization measurements using a GC4000A (GDX-403
column). The long chain hydrocarbons in the oil phase were
detected by flame ionization measurements using a GC-2010
(Rtx-1 capillary column). The inorganic gas products were
detected online by thermal conductivity measurements using
a GC4000A (carbon molecular sieves column). The H2O
and methanol products in the liquid phase were detected
by thermal conductivity measurements using a GC4000A
(GDX-401 column). The alcohol products in the liquid phase
were detected by flame ionization measurements using a
GC-7AG (Chromosorb 101).
Figure 1. XRD patterns of the fresh catalysts.
The HRTEM images (Figures 2a-c) show nanoscale
rows of metal particles in close proximity to each. As
observed, the distances between two adjacent planes, d, were
measured to be 0.24 and 0.21 nm, which are characteristic
of Cu2O and CoO phases, respectively. It is expected that
such intimate contact among the metal particles would
greatly improve the synergism of the Cu‑Co dual sites and
reduce side reactions. More HRTEM images are shown
in Supplementary Information section (Figure S2), which
could analyze the distribution of metal particles clearly.
Additionally, approximately 81% of the particles reside
within the channels of the 10Co5Cu/CNTs. In contrast, as
shown in Figure 2e, 100% of the particles were located on
Results and Discussion
Structural and morphological study of the catalysts
Figure 1 presents the XRD patterns of the fresh
catalyst samples. As shown, the characteristic CNTs peak

1376
Carbon Nanotube-Supported Copper-Cobalt Catalyst for the Production of Higher Carbon Number Alcohols
J. Braz. Chem. Soc.
the exterior surface of 10Cu5Co-out-CNTs. Furthermore,
the particle size distribution of the 10Co5Cu/CNTs
catalyst (Figure 2c), shows a size distribution of 2-4 nm
both inside and outside the nanotubes. However, the main
particles size increased to 10-20 nm for 10Cu5Co-out-CNTs.
After reaction, as shown in Figures 2d-f, grain growth
of nanoparticles were found in the samples. The most
abundant particles were in the size range of 2-30 nm for
10Co5Cu/CNTs and 10-40 nm for 10Cu5Co-out-CNTs.
It is also observed that many particles still encapsulated
within the channels of 10Co5Cu/CNTs after reaction.
Therefore, the confinement effect of carbon nanotubes may
considerably influence the distribution of metal particles,
without the confinement effect of the tubes, the cobalt and
copper particles are easily aggregated during the reaction.
The N2-physisorption isotherms of the 10Co5Cu/CNTs
catalyst are shown in Figure 3. According to the IUPAC
classification, the catalyst exhibits a type IV isotherm
having an inflection point around P/P0 = 0.4. The BJH
model was used to estimate mesopore size distributions.
As shown in Figures 3a-b, it is observed that these samples
possessed two types of mesopores, the CNT channels
present a narrow pore size distribution with a pronounced
peak at 3.4 nm, while the less uniform mesopores in
the range of 10-50 nm were assigned to the interstitial
voids among the CNTs. The BET surface area of the
10Co5Ce/CNTs and CNTs samples were 226 and
248 m2 g-1, respectively. It is indicated that all the
samples has the similar BET surface area and pore size
distribution.
Figure 2. HRTEM images of fresh (a) 10Cu5Co/CNTs; (b) 7.5Cu7.5Co/CNTs; (c) 10Co5Cu/CNTs catalyst; (e) 10Cu5Co-out-CNTs; HRTEM images of
used (d) 10Co5Cu/CNTs; (f) 10Cu5Co-out-CNTs, (600-700 particles were counted to determine the particle size distribution, and 300-400 particles were
counted to calculate the percentage of particles inside and outside of the tubes).

Vol. 29, No. 7, 2018
Wang et al.
1377
exhibited low activity and presented considerably low
CO conversion and STY. Furthermore, methanol is the
major product across alcohols formation. Conversely, the
12.5Cu2.5Co/CNTs catalyst exhibited considerably higher
catalytic performance. The CO conversion and total
alcohol STY increased by one order of magnitude relative
to that of the 15Cu/CNTs sample. This result indicates
that the addition of cobalt, even at low loading levels, has
great influence on the properties of copper, and that there
are synergistic effects between them. Additionally, the
alcohol distribution results indicate that the reaction over
CuCo/CNTs tends to favor the formation of C2+ alcohols.
Thus, both structural and electronic interactions may exist
between copper and cobalt.
As observed, the 7.5Cu7.5Co/CNTs catalyst displayed
high activity and the selectivity of ethanol reached up to
ca. 30.1%, with the selectivity for C2+ alcohols reaching
ca. 57.7%. The 10Co5Cu/CNTs catalyst displayed a larger
STY (372.9 mg gcat-1 h-1), together with a comparable higher
alcohols distribution to that of the 10Cu5Co/CNTs sample.
The 15Co/CNTs catalyst had a considerably higher activity
with a CO conversion of 48.8% and a corresponding total
alcohol selectivity of 13.4%. Further observations show
that the selectivity for alcohols decreased as a function
of increased cobalt loading in the catalysts. The excellent
C2+ alcohols selectivity and yield are obtained over the
catalyst with weight ratio of Cu/Co = 0.5-2. Taking high
C2+ alcohols selectivity, low CO2 selectivity and narrowed
alcohols distribution into consideration, carbon nanotube-
supported copper-cobalt catalyst turns to be better than
Figure 3. Nitrogen adsorption-desorption isotherms and pore size
distributions of (a) the fresh 10Co5Cu/CNTs catalyst and (b) CNTs
sample.
27
CuZnAl26 and Cu-Co/Mn2O3-Al2O3 catalyst recently
reported for higher alcohols synthesis.
Catalytic performance evaluation
It is also observed that 10Cu5Co-out-CNTs exhibited
a remarkably lower catalytic performance than that of
10Cu5Co/CNTs. The total alcohol STY decreased by
The catalytic performance of the CuCo/CNTs series
of catalysts was studied and compared. The data is
presented in Table 1.As observed, the 15Cu/CNTs catalyst
-1
163.3 mg gcat h-1, and furthermore, the total alcohols
selectivity and the alcohol distribution results showed
Table 1. Catalytic performances of CuCo-based catalysts in CO hydrogenation
Total alcohol
STY /
(mg gcat-1 h-1)
Carbon selectivity / (C mol%)
Alcohol distribution / (C mol%)
CO
Sample
conversion / %
CH4
C2-C4
ROH
CO2
MeOH EtOH
PrOH
BuOH C5OH
15Co/CNTs
48.8
40.1
38.9
32.6
28.9
21.3
3.5
256.5
266.9
372.9
318.2
292.3
230.9
21.4
68.5
61.1
48.5
43.2
44.9
50.4
30.3
60.7
–
9.1
12.1
14.0
20.4
17.6
11.1
29.7
8.7
13.4
24.5
35.8
34.2
36.3
37.0
27.1
28.6
–
9.0
2.3
1.7
2.2
1.2
1.5
12.9
2.0
–
59.2
56.8
52.7
42.3
50.0
57.8
85.1
60.2
–
25.1
25.0
26.5
30.1
28.0
25.0
9.1
10.1
10.6
12.5
16.8
13.0
11.0
4.0
5.2
7.1
7.3
9.6
7.8
5.5
1.8
4.9
–
0.4
0.5
1.1
1.3
1.2
0.7
0.1
0.5
–
12.5Co2.5Cu/CNTs
10Co5Cu/CNTs
7.5Cu7.5Co/CNTs
10Cu5Co/CNTs
12.5Cu2.5Co/CNTs
15Cu/CNTs
10Cu5Co-out-CNTs
CNTs
14.0
0
129.0
0
23.6
–
10.9
–
–
Reaction conditions: slurry reactor, 300 °C, 4.5 MPa, 10000 mL gcat-1 h-1, 2:1 H2:CO syngas ratio, TOS: 7 h; STY: space-time yield; CNT: carbon nanotube.

1378
Carbon Nanotube-Supported Copper-Cobalt Catalyst for the Production of Higher Carbon Number Alcohols
J. Braz. Chem. Soc.
that particles on the outside were not as effective for HAS
as particles inside the CNTs. In addition, the pure CNTs
sample exhibited no activity for higher alcohols synthesis
through CO hydrogenation.
XPS characterization
XPS is typically employed to analyze the composition
and oxidation states of catalyst surface species. Thus, in situ
and ex situ XPS spectra of the Cu 2p, Co 2p, C 1s and O 1s
core levels of the 10Co5Cu/CNTs and 15Co/CNTs samples
were recorded.
Correlation studies on the active sites and catalytic behavior
The Co 2p XPS spectra of the fresh and reduced
10Co5Cu/CNTs samples are shown in Figure 5a. The
Co 2p3/2 main peak was observed at ca. 780.7 eV, along
with a satellite peak at 786.7 eV. The peak with the lowest
binding energy (778.8 eV) is attributed to Co0, whereas the
peak observed at the higher binding energy (780.7 eV) is
associated with Co2+ and the peak at 780.0 eV attributed
to the Co3+ state.30-32 To confirm the distribution of the
Co species, the Co 2p XPS spectrum of the reduced
15Co/CNTs sample was measured (Figure 5c).
The measured XPS surface layer revealed relative
Co0/(Co0 + Co2+) ratios in the reduced 10Co5Cu/CNTs
and 15Co/CNTs catalysts of 2.92 and 3.61%, respectively.
Based on the experimental results, the Co2+ site is thought
to be beneficial for high alcohol selectivity, associatively
adsorption of CO on Co2+ site and coupling of CO and CHx
to form oxygenates. Similar results and discussion were
also proposed by Wang et al.33
As shown in Figure 5b, the Cu 2p3/2 spectrum of the
fresh sample is characterized by a main peak at 932.69 eV
with a satellite peak appearing at a higher binding energy,
indicating the presence of surface Cu2+.34 Conversely, the
Cu 2p3/2 spectrum of the reduced sample was characterized
by symmetrical major peaks and the absence of satellite
structures. As XPS and XAES data show, for the fresh
sample, Eb(Cu2p3/2) is located at 932.69 eV with an
Ek(CuL3VV) value of 916.38 eV, giving an Auger
parameter α′(Cu), of 1849.07 eV, which is associated with
Cu+. However, for the reduced sample, Eb(Cu2p3/2) and
Ek(CuL3VV) values are 932.89 and 918.02 eV, respectively,
giving an α′(Cu) of 1850.91 eV, which is associated with
Cu0.35 Therefore, it could be deduced that only metallic
copper is present on the surface of the catalysts after in situ
reduction.
H2-TPR characterization
The H2-TPR profiles of the fresh catalysts are presented
in Figure 4. Specifically, the 15Cu/CNTs sample exhibited
a broad peak within the temperature range of 200 to
380 °C, which was attributed to the reduction of Cu2O and
CuO based on the XRD results. The 15Co/CNTs sample
exhibited peaks centered at 292, 395 and 520 °C, which
were primarily attributed to the reduction of CoO based on
the XRD results. It was observed that the temperature of
the first reduction peak decreased gradually as a function of
increasing the Cu loading. Catalyst reduction is indicated
to be influenced by the strong mutual effect of cobalt
and copper, and the reduction of cobalt is considerably
enhanced by the presence of copper, which results in
an obvious enhancement of catalyst reducibility in our
system. The similar conclusion was also proposed by
other researchers.28,29 Hence, the first broad peak in the
CoCu/CNTs samples was attributed to the reduction of
copper and cobaltous oxide. Additionally, a broad peak
at ca. 480 °C was observed in all of these samples. To
confirm the assignment of the broad peaks, TPR-mass
spectrometry analysis was conducted and the results are
shown in Supplementary Information section (Figure S1).
It was found that a degree of cobaltous oxide was reduced
at the higher temperatures, the addition of metals and metal
oxides accelerated the decomposition of the nanotubes and
their reduction peaks overlapped with the decomposition
peak of the CNTs.
Compositional analysis to obtain Cu and Co content
of the bulk 10Co5Cu/CNTs, 7.5Cu7.5Co/CNTs and
10Cu5Co/CNTs samples is measured using inductively
coupled plasma-atomic emission spectroscopy (ICP-AES)
and data is presented in Supplementary Information section
(Table S1). Actual metal content reveals incorporation in
ratios consistent with theoretical loading values.
Combined with the characterization analysis and
experimental results, two mechanistic pathways are
assumed for the CoCu-based catalysts in relation to
the oxygenation of the generated carbon chain to form
Figure 4. H2-TPR profiles of the fresh catalysts.

Vol. 29, No. 7, 2018
Wang et al.
1379
over a short distance between the Cu0 and Co0 sites for
subsequent hydrogenation to produce higher carbon
number alcohols. These alcohols synthesis mechanism has
been termed as a CO insertion mechanism.36 The second
dual active site for alcohol synthesis is via the Co0‑Co2+
center, the surface reactions on these sites are similar to
those on the Cu0‑Co0 center. On the Co0‑Co2+ dual sites,
metallic cobalt is the active site for CO dissociation and
chain propagation, while Co2+ is the active site for CO
associative adsorption and CO insertion, therefore Co2+
sites favor high oxygenate yields. A similar route for
chain growth termination to produce alcohols was also
proposed by Baker et al.37 The metallic oxide Co pair
(Co0‑Co2+) could explain why the 15Co/CNTs sample
also produces higher carbon number alcohols during the
reaction; as shown in Table 1, the selectivity for total
alcohol reaches up to 13.4%. The proposed mechanism
is illustrated in Scheme 1.
Scheme 1. Reaction model for the formation of higher alcohols over the
CuCo/CNTs catalyst.
The Anderson-Schulz-Flory (ASF) plots (logarithm of
mole fraction vs. carbon number) for the distribution of
alcohols obtained over four catalysts are shown in Figure 6.
The carbon number distributions of alcohols obtained over
the 15Co/CNTs, 10Co5Cu/CNTs and 10Cu5Co/CNTs
catalysts display excellentASF plots. Partial deviation from
theASF distribution was observed for the 7.5Cu7.5Co/CNTs
catalyst. The appropriate Cu/Co ratio enhances the
synergistic effect of the active species and improves C2+
alcohol selectivity.
Figure 5. (a), (c) Co 2p and (b) Cu 2p XPS spectra of the fresh and
reduced catalysts (in situ reduction was conducted in H2 at 673 K for 1 h).
Conclusions
alcohols. The first route is a Cu0‑Co0 center, with Co0
acting as an active site for CO dissociation, C‑C chain
We have demonstrated that the bimetallic catalyst
with a Cu/Co ratio from 0.5 to 2 favors higher carbon
number alcohols formation during CO hydrogenation.
The 7.5Cu7.5Co/CNTs catalyst exhibited a relatively high
selectivity for the formation of ethanol (30.1%) and C2+
alcohols (57.7%) with a narrow range distribution, and
•
growth, and hydrogenation to produce CnHx groups,
whereas Cu0 is the major site for methanol synthesis
and is involved in the CO associative adsorption to
•
form CO•. The CO• moiety moves to the CnHx group
(or vice versa) and is inserted via surface migration

1380
Carbon Nanotube-Supported Copper-Cobalt Catalyst for the Production of Higher Carbon Number Alcohols
J. Braz. Chem. Soc.
Figure 6. ASF plots for the distribution of alcohols obtained over the investigated catalysts.
the 10Co5Cu/CNTs catalyst displayed a high alcohol STY
(372.9 mg gcat-1 h-1).
results) are available free of charge at http://jbcs.sbq.org.br
as PDF file.
The superior confinement effect allows intimate
contact between the copper-cobalt active species in the
nanotubes, which improves their geometric interactions and
synergistic effect. The synergistic effect between copper
and cobalt is crucial for optimal product distribution.
Metallic copper alone shows negligible catalytic activity
for CO hydrogenation, however, in the presence of
Cu0‑Co0 dual catalytic sites, Cu is effective, as the sites
for the oxygenation of the generated carbon chains form
alcohols, in addition to enhancing cobalt reduction.
Furthermore, the Co0–Co2+ center is also beneficial to
HAS, the Co2+ site terminating the carbon chain growth
to produce alcohols and favor alcohols yields. The ratio of
Cu/Co considerably influences metal particle properties
and it likely plays an important role in determining the
selectivity towards higher alcohols.
Acknowledgments
This work was supported by the National Natural
Science Foundation of China (grant No. 21573269),
the Natural Science Foundation of the Jiangsu Higher
Education Institutions of China (grants No. 17KJB150038
and 17KJB530010), the Research Project of Xuzhou
Institute of Technology, China (grant No. XKY2016114),
and Natural Science Foundation of Jiangsu Province, China
(grants No. BK20171168 and BK20171169).
References
1. Luk, H. T.; Mondelli, C.; Ferré, D. C.; Pérez-Ramírez, J.; Chem.
Soc. Rev. 2017, 46, 1358.
2. Gangwal, V.; Gangwal, S. K.; Energy Fuels 2008, 22, 814.
3. Wang, P.; Chen, S. Y.; Bai, Y. X.; Gao, X. F.; Sun, K.; Han, Y.
Z.; Tan, Y. S.; Fuel 2017, 195, 69.
Supplementary Information
Supplementary data (H2-TPR-MS, HRTEM and ICP
4. Gupta, M.; Smith, M. L.; Spivey, J. J.; ACS Catal. 2011, 1, 641.

Vol. 29, No. 7, 2018
Wang et al.
1381
5. Wang, J. J.; Chernavskii, P. A.; Khodakov, A. Y.; Wang, Y.;
J. Catal. 2012, 286, 51.
23. Guan, J.; Pan, X. L.; Liu, X.; Bao, X. H.; J. Phys. Chem. C
2009, 113, 21687.
6. Fang, K. G.; Li, D. B.; Lin, M. G.; Xiang, M. L.; Wei, W.; Sun,
Y. H.; Catal. Today 2009, 147, 133.
24. Guo, S.; Pan, X.; Gao, H.;Yang, Z.; Zhao, J.; Bao, X. H.; Chem.
- Eur. J. 2010, 16, 5379.
7. Medford, A. J.; Lausche, A. C.; Temel, B.; Schjodt, N. C.;
Norskov, J. K.; Top. Catal. 2014, 57, 135.
25. Su, J.; Zhang, Z.; Fu, D.; Jiang, Z.; Xu, J.; Han, Y. F.; J. Catal.
2016, 336, 94.
8. Prieto, G.; Beijer S.; Smith, M. L.; Au, Y.; Wang, Z.; Angew.
Chem., Int. Ed. 2014, 53, 6397.
26. Liu, Y.; Deng, X.; Han, P.; Huang, W.; Fuel Process. Technol.
2017, 167, 575.
9. Xiaoding, X.; Doesburg, E. B. M.; Scholten, J. J. F.; Catal.
Today 1987, 2, 125.
27. Zhao, L.; Li, W.; Zhou, J.; Mu, X.; Fang, K.; Int. J. Hydrogen
Energy 2017, 421, 7414.
10. Smith, K. J.; Anderson, R. B.; J. Catal. 1984, 85, 428.
11. Xiao, K.; Bao, Z. H.; Fang, K. G.; Lin, M. G.; Sun,Y. H.; Chin.
J. Catal. 2013, 34, 116.
28. Wang, J. J.; Chernavskii, P. A.; Khodakov, A.Y.; Wang, Y.;
J. Catal. 2012, 286, 51.
29. Fierro, G.; Jacono, M. L.; Inversi, M.; Dragone, R.; Porta, P.;
Top. Catal. 2000, 10, 39.
12. Subramanian, N. D.; Balaji, G.; Kumar, C.; Spivey, J. J.; Catal.
Today 2009, 147, 100.
30. Reinikainen, M.; Niemelä, M. K.; Kakuta, N.; Suhonen, S.;
Appl. Catal., A 1998, 174, 61.
13. Khodakov, A. Y.; Griboval-Constant, A.; Bechara, R.;
Zholobenko, V. L.; J. Catal. 2002, 206, 230.
14. Borg, Ø.; Eri, S.; Blekkan, E. A.; Storsæter, S.; Wigum, H.;
Rytter, E.; J. Catal. 2007, 248, 89.
31. Guczi, L.; Bazin, D.; Kovács, I.; Borkó, L.; Schay, Z.; Lynch,
J.; Top. Catal. 2002, 20, 129.
32. Ernst, B.; Bensaddik, A.; Hilaire, L.; Chaumette, P.;
Kiennemann, A.; Catal. Today 1998, 39, 329.
33. Wang, Z.; Kumar, N.; Spivey, J. J.; J. Catal. 2016, 339, 1.
34. Wang, J.; Chernavskii, P. A.; Wang, Y.; Khodakov, A. Y.; Fuel
2013, 103, 1111.
15. Khodakov, A. Y.; Bechara, R.; Griboval-Constant, A.; Appl.
Catal., A 2003, 254, 273.
16. Karaca, H.; Safonova, O. V.; Chambrey, S.; Fongarland, P.;
Roussel, P.; J. Catal. 2011, 277, 14.
17. Karaca, H.; Hong, J.; Fongarland, P.; Roussel, P.; Lacroix, M.;
Chem. Commun. 2010, 46, 788.
35. Wang, P.; Huang, W.; Zhang, G.; Gao, Z.; Tang, Y.; Sun, K.;
J. Ind. Eng. Chem. 2015, 26, 243.
18. den Otter, J. H.; Nijveld, S. R.; de Jong, K. P.; ACS Catal. 2016,
6, 1616.
36. Fang, K.; Li, D.; Lin, M.; Xiang, M.; Wei, W.; Sun, Y.; Catal.
Today 2009, 147, 133.
19. Deng, D.;Yu, L.; Chen, X.; Wang, G.; Jin, L.; Pan, X.; Angew.
Chem., Int. Ed. 2013, 52, 371.
37. Baker, J. E.; Burch, R.; Hibble, S. J.; Loader, P. K.; Appl. Catal.
1990, 65, 281.
20. Chen, W.; Fan, Z.; Pan, X.; Bao, X.; J. Am. Chem. Soc. 2008,
130, 9414.
Submitted: October 3, 2017
21. Chen, W.; Pan, X. L.; Willinger, M. G.; Su, D. S.; Bao, X. H.;
J. Am. Chem. Soc. 2006, 128, 3136.
Published online: December 20, 2017
22. Pan, X.; Fan, Z.; Chen, W.; Ding, Y.; Luo, H.; Bao, X.; Nat.
Mater. 2007, 6, 507.
This is an open-access article distributed under the terms of the Creative Commons Attribution License.