Effect of the dimensions of carbon nanotube channels on copper-cobalt-cerium catalysts for higher alcohols synthesis

Article abstract of DOI:10.1016/j.catcom.2015.12.012

A series of carbon nanotube (CNT)-supported copper-cobalt-cerium catalysts were prepared and investigated for higher alcohols synthesis. The superior selectivity for the formation of ethanol and C_{2 +} alcohols achieved using the CuCoCe/CNT(8) catalyst was 39.0% and 67.9%, respectively. The diameters of CNTs considerably influence the distribution of metal particles and the electronic interaction between the tube surface and the active species. The electronic effect between the encapsulated Co species and the inner surface is greatly improved in the narrowest CNT channel, which is expected to facilitate the reduction of cobaltous oxide and promote the alcohols yield remarkably (291.9 mg/g_cath).

Full text of DOI:10.1016/j.catcom.2015.12.012

Catalysis Communications 75 (2016) 92–97
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Effect of the dimensions of carbon nanotube channels on
copper–cobalt–cerium catalysts for higher alcohols synthesis
Peng Wang a,b, Yunxing Bai , He Xiao a,b, Shaopeng Tian , Zhenzhou Zhang a,b, Yingquan Wu a,
a,b
a,b
a
a
a
a,
Hongjuan Xie , Guohui Yang , Yizhuo Han , Yisheng Tan ⁎
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
University of Chinese Academy of Sciences, Beijing 100049, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 October 2015
Received in revised form 10 December 2015
Accepted 12 December 2015
Available online 14 December 2015
A series of carbon nanotube (CNT)-supported copper–cobalt–cerium catalysts were prepared and investigated
for higher alcohols synthesis. The superior selectivity for the formation of ethanol and C2+ alcohols achieved
using the CuCoCe/CNT(8) catalyst was 39.0% and 67.9%, respectively. The diameters of CNTs considerably
influence the distribution of metal particles and the electronic interaction between the tube surface and the ac-
tive species. The electronic effect between the encapsulated Co species and the inner surface is greatly improved
in the narrowest CNT channel, which is expected to facilitate the reduction of cobaltous oxide and promote the
alcohols yield remarkably (291.9 mg/gcath).
Keywords:
CO hydrogenation
Higher alcohol synthesis
Carbon nanotube
CuCoCe
© 2015 Elsevier B.V. All rights reserved.
1
. Introduction
Higher alcohols synthesis (HAS) from hydrogenation of carbon
enrichment of reactants inside CNTs [15], and electronic interaction of
the confined materials with CNTs [16] can be achieved. Furthermore,
the properties of the active components can be tuned by varying the
location on the exterior or interior surface of the CNTs, and the diameter
of the CNT channels. To date, there are no reports and research on the
influence of the diameters of CNTs on Cu–Co systems employed for
HAS. Thus, CuCoCe/CNT catalysts with different CNT channel sizes
were prepared and investigated, and the catalytic performance of the
catalysts toward CO hydrogenation was examined in both a fixed bed
and a slurry reactor.
monoxide has attracted significant interest because of its potential
application as a promising route for the production of fuel blends and
hydrogen energy carriers or as value-added chemicals in fine chemical
synthesis [1,2]. Typically, modified FT catalysts (e.g., Cu–Co) are
regarded as one of the most promising candidates for HAS [3–6]. How-
1 6
ever, these catalytic systems produce a wide distribution of C –C linear
alcohols, which obeys the Anderson–Schulz–Flory (ASF) distribution,
thus leading to the formation of methanol as the major product
among the alcohols formed [7–9]. Therefore, development of catalysts
with high efficiency and selectivity for the short chain alcohols
2. Experimental
(
especially for ethanol) are highly desirable and challenging.
2.1. Materials
To narrow the alcohols distribution and improve the selectivity for
short chain alcohols, a suitable promoter and support are necessary.
Ceria exhibits interesting redox properties, and it has been reported
Analytical grade chemicals, including Cu(NO
3 2 2 3 2 2
) ·3H O, Co(NO ) ·6H O,
and Ce(NO ·6H O, were purchased from the Beijing Chemical Co.,
3
)
3
2
that partially reduced CeO
2
is expected to create new sites for the ad-
Ltd., and used without further purification. Three types of CNTs were
used: CNT(8) (o.d.: b8 nm), CNT(20) (o.d.: 10–20 nm), CNT(30) (o.d.:
20–30 nm); they were purchased from Chengdu Organic Chemicals.
HRTEM analysis shown that the average inner diameter of CNT(8),
CNT(20) and CNT(30) was around 3.9, 6.3 and 7.4 nm respectively.
The wall thickness of them was in range of 1.5–3 nm, 3–7 nm and
sorption of CO, thereby favoring the breaking of the C–O bond [10]. In
particular, the Co–CeO2−x interface could be beneficial for the forma-
tion of alcohols in FT reactions [11,12]. Therefore, cerium is chosen as
the third component in the proposed Cu–Co–Ce catalyst. Carbon
nanotubes (CNTs) have been drawing increasing attention since their
discovery [13]. The confinement effects in catalysis have been systemat-
ically researched. Previous studies concluded that space restriction [14],
3
7–12 nm respectively. Raw CNTs were refluxed in 68 wt.% HNO 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
3
7 wt.% HNO
3
for 5 h at 100 °C. Then, the mixture was filtered and
⁎
Corresponding author.
E-mail address: tan@sxicc.ac.cn (Y. Tan).
washed with deionized water, followed by drying at 60 °C for 12 h.
http://dx.doi.org/10.1016/j.catcom.2015.12.012
566-7367/© 2015 Elsevier B.V. All rights reserved.
1

P. Wang et al. / Catalysis Communications 75 (2016) 92–97
93
Table 1
Catalytic performances of CuCoCe/CNT catalysts in carbon monoxide hydrogenation.
Samples
Reactor
CO conv.(%)
Total alc. STY (mg/gcath)
Carbon selectivity (C mol%)
Alcohol distribution (C mol%)
CH
4
C
2
–C
4
ROH
CO
2
MeOH
EtOH
PrOH
BuOH
5
C OH
CuCoCe-out-CNT(8)
CuCoCe/CNT(8)
Fixed bed
Fixed bed
Slurry reactor
Fixed bed
Fixed bed
17.6
28.3
26.1
19.1
18.0
191.7
291.9
272.6
190.1
185.0
28.3
27.6
22.5
31.9
33.0
39.3
34.3
34.1
34.8
33.7
29.7
34.5
38.8
27.3
26.6
2.7
2.7
4.5
4.1
5.2
43.6
37.3
32.1
44.0
48.3
32.6
34.8
39.0
30.7
25.5
15.1
18.2
18.0
15.1
16.3
7.3
8.3
9.0
8.6
8.0
1.4
1.5
1.9
1.5
2.0
CuCoCe/CNT(20)
CuCoCe/CNT(30)
2
Reaction conditions: 300 °C, 4.5 Mpa, 10,000 mL/gcath, 2:1 H : CO syngas ratio, TOS: 7 h for fixed bed, 12 h for slurry reactor.
2
.2. Synthesis of Cu/Co/Ce-based catalysts
The CNTs were immersed in an ethanolic solution of precursor salts
displayed comparable space-time yield (STY). In contrast, CuCoCe/
CNT(8) exhibited a much larger space-time yield (291.9 mg/gcath),
and the C2+ alcohols selectivity reached ~62.7%, which is considerably
higher than that achieved over CuCoCe/CNT(30) and CuCoCe/CNT(20).
It is also observed that CuCoCe-out-CNT(8) exhibited a remarkably
lower catalytic performance than that of CuCoCe/CNT(8). The total alco-
hol STY decreased by 100 mg/gcath, and furthermore, the total alcohols
selectivity and the alcohol distribution results showed that particles on
the outside were not as effective for HAS as particles inside the CNTs.
The catalytic performance of CuCoCe/CNT(8) in a slurry reactor was
also studied. As the results show, the catalyst tends to favor the forma-
tion of higher alcohols in a slurry reactor, the C2+ alcohols selectivity
increased to 67.9% and the ethanol selectivity reached ~39%, which
were considerably higher than those achieved over other catalysts.
of Cu, Co, and Ce, and then subjected to ultrasonic treatment and stir-
ring. The ultrasonic treatment and stirring facilitated the filling of the
CNT channels with the precursor solution during impregnation. After
1
0 h of impregnation, the mixture was dried at 60 °C overnight, follow-
ed by heating to 110 °C for 10 h. After drying, the products were calcined
in N at 673 K for 4 h with a heating rate of 2 K/min. A series of catalysts
2
were accordingly prepared using this method, i.e., CuCoCe/CNT(8),
CuCoCe/CNT(20), CuCoCe/CNT(30), and the active components sup-
ported on CNT(8) with closed caps were denoted as CuCoCe-out-
CNT(8). The loading of Cu, Co, and Ce was controlled at 10, 5, and
5
wt.% respectively.
2
.3. Characterization
Powder X-ray diffraction patterns (XRD) were recorded on a Rigaku
MiniFlex II X-ray diffractometer. H
tion mass spectrometry analysis (H
2
-temperature-programmed reduc-
-TPR-MS) was carried out to study
2
reducibility of the catalysts with chemisorption instrument (TP-5080)
and OmniStar instrument. X-ray photoelectron spectroscopy (XPS)
was used to analyze the change of surface composition measured by
AXIS ULTRA DLD equipment. The morphology and microstructure of
the materials were observed on a high resolution transmission electron
microscope (HRTEM, JEOL, JEM-2100F).
2
.4. Catalysis studies
Carbon monoxide hydrogenation reactions were performed both in
a fixed bed stainless steel tubular microreactor (15.5 mm in diameter,
00 mm in length) and in a 0.5 L continuous slurry reactor with a me-
chanical agitator. The 30–40 mesh silica sand (SiO ) was used to dilute
5
2
the 1.5 g catalyst particles in the fixed bed. The liquid paraffin with the
distill-off temperature at 340 °C was used as solvent in the slurry reac-
tor. The reactions were run at temperature 573 K and pressure
4
2
.5 MPa, using a feed gas of 2:1 H and CO mixture at a space velocity
of 10,000 mL/gcath. Catalysts were reduced with a 10% hydrogen-in-ni-
trogen mixture for 3 h at 673 K in the fixed bed or in a tube furnace. All
experimental data were obtained under steady-state conditions that
were invariably maintained for 7 h, and no deactivation was observed
in 24 h. The products were analyzed by four online GCs during the
reaction.
3
. Results and discussion
3
.1. The evaluation of the catalytic performance
The catalytic performance of CuCoCe/CNT(8) was studied and com-
pared with that of CuCoCe/CNT(20), CuCoCe/CNT(30), and CuCoCe-out-
CNT(8). The data are presented in Table 1. As observed, the total alco-
hols and ethanol selectivity gradually increased upon decreasing CNT
channel diameter. However, CuCoCe/CNT(30) and CuCoCe/CNT(20)
Fig. 1. XRD patterns of the (a) fresh and (b) used catalysts.

94
P. Wang et al. / Catalysis Communications 75 (2016) 92–97
Fig. 2. HRTEM images of fresh (a–b) CuCoCe/CNT(8), (c–d) CuCoCe/CNT(20), (e–f) CuCoCe/CNT(30), (g–h) CuCoCe-out-CNT(8), HRTEM images of used (A) CuCoCe/CNT(8), (C) CuCoCe/
CNT(20), (E) CuCoCe/CNT(30), (G) CuCoCe-out-CNT(8). (600–700 particles were counted to determine the particle size distribution, and 300–400 particles were counted calculate the
percentage of particles inside and outside of the tubes.)
3
3
.2. Characterization
CNT peak was observed at 2θ 25.8°. After reaction, as shown in Fig. 1b,
Cu (JCPD No. 04-0836) became the principal phase in the all samples.
0
.2.1. Structural and morphological study of the catalysts
Fig. 1a presents the XRD patterns of the fresh catalyst samples. As
The HRTEM images (Fig. 2a–h) show that the Cu, Co, and Ce particles
were uniformly distributed in the fresh catalysts. As observed in Fig. 2b,
the metal particles were arranged in a row and were in close proximity
to each other on the nanoscale. Additionally, approximately 80% of the
particles resided in the tube channels of CuCoCe/CNT(8). In contrast,
as shown in Fig. 2c–f, these particles were distributed both inside and
outside the tubes of CuCoCe/CNT(20) and CuCoCe/CNT(30), and
observed, the samples mostly consisted of a Cu
corresponding peaks at 2θ 36.5, 42.4, 61.5, 73.7, and 77.6° (JCPD No.
5-3288). Additionally, no distinct peaks corresponding to cobalt
2
O phase, with
6
species were observed. Therefore, it is hypothesized that the Co compo-
nent is homogeneously dispersed in the catalysts. The characteristic

P. Wang et al. / Catalysis Communications 75 (2016) 92–97
95
Fig. 3. Nitrogen adsorption–desorption isotherms and pore size distributions of fresh and used catalysts.
approximately 72% and 59% of the particles were encapsulated within
the CNT channels, respectively. This finding indicated that the particles
preferred to stay in the CNT channels as the tube inner diameter
decreased. In addition, as shown in Fig. 2g–h, 100% of the particles
were located on the exterior surface of CuCoCe-out-CNT(8). The particle
size distribution of the fresh samples is shown in Fig. 2a, c, e, g. It is
found that most abundant particles were in the size range of 2–4 nm
for first three samples; however, the main particle size increased to
Halenda (BJH) model was used to estimate the mesopore size distribu-
tions of all samples. As shown in Fig. 3b–d, it is observed that these
samples possessed two kinds of mesopores, the channels of CNTs had
very narrow pore size distribution with the pronounced peak at 3.4 nm,
while the less uniform mesopores in the range of 4–30 nm were assigned
to the interstitial holes among carbon tubes. After reaction, the pore size
of CNT channels remained constant; however the pore size distribution of
interstitial holes tended to become wider. More detailed textural analysis
was shown in Table 2. It is evidenced that pore volume of
CNT(8) channels decreased sharply (Fig. 3b), and the BET surface areas
decreased by more than half after reaction. The fact reveals that many
encapsulated particles still existed in CNT(8) channels and grew up;
they were not easy to move out during the reaction. However, the
particles were facilitated to migrate to the outside surface of tubes for
other samples. In addition, these interstitial holes may prevent the
outside nanoparticles from severe agglomeration. Combined with
evaluation result, it is speculated that the small encapsulated metals
were more effective and beneficial to HAS than the larger and outside
ones.
1
0–25 nm for CuCoCe-out-CNT(8). After reaction, as shown in Fig. 2A,
C, E, G, grain growth of nanoparticles was found in all samples. However,
the most abundant particles were in the size range of 2–30 nm for
CuCoCe/CNT(8) and CuCoCe/CNT(20), and 10–40 nm for CuCoCe/
CNT(30) and CuCoCe-out-CNT(8). It is observed that many particles
still encapsulated within the CNT(8) channels after reaction, and
fewer particles resided in the tube channels of CNT(20), nearly all
particles were in outside tubes of CNT(30).
2
The N -physisorption isotherms of CNT(8) and CuCoCe/CNT(8) were
shown in Fig. 3a; according to the IUPAC classification the isotherms were
a type IV with an increase around P/P = 0.4. The Barrett–Joyner–
0
Table 2
Textural properties of the CuCoCe/CNT catalysts.
Sample
Before reaction
After reaction
2
3
2
3
BET surface area (m /g)
Pore volume (cm /g)
BET surface area (m /g)
Pore volume (cm /g)
CNT(8)
248
222
232
174
226
0.56
0.46
0.97
0.51
0.52
–
–
CuCoCe/CNT(8)
CuCoCe-out-CNT(8)
CuCoCe/CNT(20)
CuCoCe/CNT(30)
101.0
179.4
173.1
180.0
0.48
0.89
0.73
0.52

96
P. Wang et al. / Catalysis Communications 75 (2016) 92–97
the existence of an interaction between the cobalt nanoparticles and the
tube surface, and the interaction becomes significantly strong in the
narrowest tubes. In contrast, the Co 2p3/2 binding energy decreased to
7
80.7 eV upon transfer of all the particles to the exterior tube surface.
The finding indicated that the chemical state of the Co species encapsu-
lated within the CNT channels is remarkably different in comparison
to that of the Co species on the exterior tube surface. It is reported
that π-electron density is shifted from the concave inner to the convex
outer surface, and the shift is enhanced as the CNT cross-section
becomes smaller [19,20]. Therefore, the interaction between the
electron-deficient concave surface of the nanotube and the anionic
oxygen in CoO could lead to a weakened bonding strength of Co–O
and the reduction of CoO nanoparticles is facilitated considerably within
2
the smaller CNT inner tubes [21,22]. The characterization of H -TPR
confirms that the reduction of metal oxides is more facile upon the
decrease of the inner diameter. The reduction temperature of CuCoCe/
CNT(8) was 13 °C lower compared with that of CuCoCe/CNT(30) and
CuCoCe/CNT(20) (Fig. S1).
Furthermore, it is reported that the Co–CeO2−x interface could be
suitable for the formation of alcohols in FT reactions [11,12]. The relative
surface concentrations of cobalt and cerium were calculated for the
fresh samples. Samples CuCoCe/CNT(30), CuCoCe/CNT(20), and
CuCoCe/CNT(8) displayed a Co/Ce value of 1.09, 1.16, and 1.41, respec-
tively. The results indicated that more Co–CeO2−x interfaces may be
formed in CuCoCe/CNT(8) after reduction, and the interface accelerates
x
the dissociation of the C–O bond and provides more CH intermediates.
Therefore, it remarkably increases the rate of the first C–C bond forma-
tion in the nanotube channels and accelerates the formation of higher
alcohols.
3.2.3. ASF plots
The ASF plots (logarithm of mole fraction vs. carbon number) for the
distribution of alcohols obtained over the four studied catalysts are
shown in Fig. 5. As shown in Fig. 5, the carbon number distributions of
alcohols obtained over CuCoCe/CNT(30) obeyed the ASF distribution.
Deviation from the ASF distribution was observed for CuCoCe/
CNT(20). When these active components were introduced to the
smallest CNT(8) tubes, it was clear that methanol showed a negative de-
viation and ethanol showed a positive deviation from the ASF behavior.
Furthermore, in the slurry reactor, the selectivity of methanol decreased
considerably and the selectivity of ethanol increased remarkably; thus,
ethanol became the major product among the resulting alcohols.
Based on the characterization analysis of the catalysts discussed above,
it is clear that different carbon tubes (inner diameter and wall thick-
ness) could modify the geometric and electronic interactions among
copper–cobalt–ceria active sites, which brings the deviations from the
ASF behavior, and the slurry reactor may be more favor for HAS than
fixed bed.
Fig. 4. (a) Cu 2p and (b) Co 2p XPS spectra of the fresh catalysts.
3
.2.2. XPS
XPS is typically employed to analyze the composition and oxidation
states of catalyst surface species. As shown in Fig. 4a, the Cu 2p3/2
spectra of the four fresh catalysts were characterized by asymmetrical
major peaks and satellite structures. Two peaks at about 932.5 and
9
34.6 eV were attributed to Cu+ and Cu2+ oxides, respectively,
and the peak at approximately 942.8 eV was assigned to the
2
+
satellite peak of Cu oxidation states [17]. It was found that the relative
2
+
area/intensity of the profiles of the Cu
oxides increased upon
4. Conclusions
increasing the CNT wall thickness; the values of Cu2 /(Cu +Cu
+
2+
+
)
are 29.8%, 46.7% and 62.1% for CuCoCe/CNT(8), CuCoCe/CNT(20),
CuCoCe/CNT(30) respectively, and 33.1% for the CuCoCe-out-
CNT(8) sample. The analysis suggested that the interaction
between metals and CNTs increased upon decreasing wall
thickness of carbon tubes and CNT(8) had the strongest interaction
with copper, which prevented most of the cuprous oxide from forming
copper oxide.
The Co 2p XPS spectra of the four fresh samples are shown in Fig. 4b.
The Co 2p3/2 main peak was observed at ~780.7 eV, along with a satellite
peak at 786.7 eV. The peak observed at 780.7 could be associated with
the Co2+ component [18], and Co 2p3/2 binding energy of CuCoCe/
CNT(8) shifted to a high energy of 0.6 and 0.5 eV relative to that of
CuCoCe/CNT(20) and CuCoCe/CNT(30), respectively. This shift indicates
We have demonstrated that ternary CuCoCe/CNT(8) catalyst
displayed a rather high selectivity for the formation of ethanol
(39.0%) and C2+ alcohols (67.9%) with a narrow range distribution
during CO hydrogenation. The smaller channels have a superior
space restriction and the thinner walls have stronger electronic
interaction between the tube surface and the confined material.
Encapsulated particles, facilitated reduction of CoO species, and
high concentrations of Co/Ce could be obtained in the narrowest
CNT channels, which contributes to the remarkable enhancement
in space-time yield and selectivity of higher alcohols. The catalytic
CO hydrogenation results show that the ternary catalyst may have
potential for scale-up in industrial applications to produce higher
alcohols.

P. Wang et al. / Catalysis Communications 75 (2016) 92–97
97
Fig. 5. ASF plots for the distribution of alcohols obtained over the four catalysts studied.
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