Synthesis of higher alcohols by the Fischer-Tropsch reaction over activated carbon supported CoCuMn catalysts

Article abstract of DOI:10.1039/c5ra10804g

Activated carbon supported CoCuMn catalysts prepared by incipient co-impregnation were studied using the Fischer-Tropsch reaction, XRD, HR-TEM, and H₂-TPR techniques. The results showed that good activities and selectivities towards C₁-C₁₄ alcohols can be obtained with the fractions of C₂-C₅ alcohols up to 80% in total alcohol distribution. It was found that increasing the Cu/Co composition ratio resulted in the enhancement of alcohol selectivity. Furthermore, our characterizations revealed that large Cu nano-particles were decorated by small spherical Co nano-particles; moreover, we speculate that such constructions were responsible for the favored formation of C₂₊ alcohols. It was also considered that smaller cobalt particles probably resulted in a greater number of Co⁰ species and thus contributed to the improvement of CO conversion.

Full text of DOI:10.1039/c5ra10804g

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Page 1 of 26
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Synthesis of higher alcohols by FischerꢀTropsch reaction over activated
carbon supported CoCuMn catalysts
Yanpeng Peia,*, Siping Jianb, Yuanyuan Chena, Chuncheng Wanga
a Guangzhou Pysynchem. Co. Ltd., Guangzhou 510640, China
b School of Chemistry and Chemical Engineering, South China University
of Technology, Guangzhou 510640, China
Abstract
Activated carbon supported CoCuMn catalysts prepared by
incipient coꢀimpregnation were studied by using FischerꢀTropsch reaction,
XRD, HRꢀTEM, and H2ꢀTPR techniques. The results showed that good
activities and selectivities towards C1ꢀC14 alcohols can be obtained with
the fractions of C2ꢀC5 alcohols up to 80% in total alcohols. It is found that
increasing the Cu/Co composition ratio resulted in the enhancement of
alcohol selectivity. Furthermore, our characterizations revealed that large
Cu nanoꢀparticles were decorated by small, spherical Co nanoꢀparticles,
which we speculate that such construction were responsible for the
favored formation of C2+ alcohols. It is also considered that smaller cobalt
particles probably resulted in more number of Co0 and thus contributed to
the improvement of CO conversion.
Keywords: Higher alcohol; FischerꢀTropsch reaction; CoCuMn;
Introduction
The FischerꢀTropsch (FT) synthesis, which produces hydrocarbons

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with oxyꢀcompounds as byproducts from coal, natural gas and biomass
via syngas intermediate, has long been known and has received renewed
attentions in recent years due to environmental concerns and energy
issues, because the products i.e. hydrocarbons and oxyꢀcompounds can
serve as alternative fuels and valuable chemical feedstock to petroleum
refining [1]. Many transitional metals such as Fe, Co, Ni, and Ru can be
used as catalysts for FT synthesis [2,3]. Among these catalysts, supported
Co catalysts appear as one of the most promising candidates for FT
synthesis because of their workable cost, high FT activity, high selectivity
towards longꢀchain hydrocarbons and low water gas shift activity [1,4]. It
is well recognized that the active site for CO dissociation, carbon chain
growth and hydrocarbon formation is metallic Co phase, whereas the
production of alcohols usually requires synergy between proximate
catalytic sites as both the dissociative sites and the nonꢀdissociative
insertion of CO. For instance, dual metal catalysts such as CuCo, CuFe
and RuMo based catalysts have been regarded as materials capable of
providing the necessary synergic actions [5,6].
Recently, oneꢀpot production of long chain αꢀalcohols (i.e. alcohols
with carbon number larger than six) via FT reaction over Coꢀbased
catalysts has aroused particular interest, owing to that these products are
used as intermediates in the synthesis of plasticizers, detergents and
lubricants. For instance, Xiang et al. reported that ternary “CoCuMn”

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metal catalysts could be tuned to strongly favor product of straight C1ꢀC14
terminal alcohols [7,8]. Alkali metals modified unsupported and SiO2
supported Co catalysts have been reported to be selective for the
production of C1ꢀC16 alcohols as well [9]. However, high alcohol
selectivity was achieved at the expense of activity for the former case
while there was very high CO2 selectivity for the latter case. In a pervious
study, activated carbon (AC) supported Co (Co/AC) catalysts have been
reported to be able to produce C1ꢀC18 linear alcohols in the presence of
Co and in situ formed Co2C phases, which were supposed to play a
pivotal role in alcohol formation during FT reaction. It is further revealed
that addition of La2O3 into the Co/AC catalysts could provide additional
site for alcohol formation and thereby improved alcohol selectivity [10].
Also, to increase the activity of Co/AC catalysts, methods to decrease the
size of Co particles are suggested to be adopted [11]. More importantly, it
is demonstrated that the Co/AC catalysts have many inherent advantages,
such as that the catalysts are tunable to increase alcohol selectivity and
activity simultaneously, and to generate low production of methane, CO2
and methanol [10ꢀ12]. These results indicate that the Co/AC catalysts
appear to be promising candidates for further modification to meet the
necessity for largeꢀscale industrial production of higher alcohols.
In the present study, we reported a study of AC supported CoCuMn
(CoCuMn/AC) catalysts for the synthesis of higher alcohols from syngas

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via FT reaction under the consideration listed as follows. (1) AC
supported Co catalysts can be anticipated to produce alcohols with little
methanol fractions; (2) Cu metal can serve as a site in favor of the
production of alcohols; (3) Mn species were introduced into the
asꢀprepared catalysts as a promoter to improve catalyst performance. The
performance data of all the catalysts were correlated with the results of
TPR, XRD and HRꢀTEM characterizations.
Experimental
Catalyst preparation
AC made from coconut shells was purchased from Tangshan Union
Activated Carbon Co. Ltd.. Before use, the material was washed with
deꢀionized water for several times to eliminate the mineral impurities [13],
followed by crushed into 80ꢀ100 meshes. A Co/AC catalyst used for
reference was prepared by the incipient wetness impregnation of AC with
an aqueous solution of cobalt nitrate (Sinopharm Chemical Reagent Co.
Ltd.). The wet sample was dried at 333 K for 6 h in air and finally calcined
at 523 K for 4 h under pure Ar. The obtained catalyst was denoted as 15Co.
CoCuMn/AC catalysts were prepared by impregnating the AC with
mixed solution of cobalt, cupric, and manganous nitrate (Sinopharm
Chemical Reagent Co. Ltd.), dried and calcined under the same conditions
as mentioned above. The obtained catalysts were denoted here as
15Co4Cu4Mn, 15Co8Cu8Mn and 8Co8Cu8Mn, respectively. For

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comparison, an AC supported CoCu biꢀmetal catalyst was prepared using
the same procedure as described above after impregnating the AC with
mixed solution of cobalt and cupric nitrate (Sinopharm Chemical Reagent
Co. Ltd.), and denoted as 15Co4Cu. The numbers adopted for catalyst
nomenclature indicate the mass percentages of each metal component
behind them relative to the initial mass of the support material in the
dried catalyst.
Catalytic test
A fixed bed reactor was used for FT synthesis. Catalysts (4 ml,
diluted with quartz sand to a volume ratio of 1:1) were in situ pretreated
in a flow of H2 (60 ml/min) at 703 K for 4 h at normal pressure. After the
catalysts were cooled to room temperature, the reactor was purged with
syngas (H2/CO = 2:1) and then charged with syngas at 3.0 MPa and a
flow rate of 500 hꢀ1, and the temperature was increased at a 0.3 K/min
heating rate to 493 K. The reaction effluent passed through a condenser to
collect the liquid products. The moment when the reactor reached the
designed temperature was taken as starting time. After 24 h stabilization,
the liquid organic products and the aqueous products for another 24 h were
offꢀline analyzed on HPꢀ6890 GC with 5% PH ME capillary columns and
FID detector after being carefully separated. The outlet gas was onꢀline
analyzed by HPꢀ6890 GC with a ParapackꢀQ column and TCD detector.
Catalyst characterization

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The crystalline phases of catalysts after reduction and reaction were
examined by Xꢀray diffraction (XRD) with Cu Kα1 radiation on a
PANalytical X’Pert PRO diffractometer at 40 kV and 40 mA. The spectra
were recorded from 20 to 60° at a scanning rate of 10°/min. For the
measurements, the catalysts were ground to fine powder and placed inside
a dish.
H2ꢀTPR experiments were carried out on an Altamira Instruments
temperature programmed system (AMIꢀ200). The sample (50 mg) was
treated by a 10% H2/Ar gas mixture in a flow rate of 50 ml/min, and the
reduction temperature was increased from room temperature to 1123 K at a
heating rate of 10 K/min. The hydrogen consumption was recorded an
online with a mass spectrometer (MS) (Oministar, Pfeiffer Vacuum).
The morphology and crystalline nature of the samples were studied
by highꢀresolution transmission electron microscopy (HRꢀTEM, C/M300
from Philips), operating at 300 kV. A few droplets of a suspension of the
calcined catalysts in ethanol were put on a microꢀgrid carbon polymer
supported on a copper grid and allowed to dry at room temperature for
HRꢀTEM observations.
3. Results and discussion
3.1. The performance of FT reaction
Table 1 shows the results of catalytic tests for the 15Co catalyst and
the CoCuMn/AC catalysts in terms of CO conversion, selectivity and

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alcohol distribution. It is seen that the activity (CO conversion)
significantly increased when 4% Cu and 4% Mu were added into the
15Co catalyst. A further increase of Cu and Mn with equal contents only
results in a slight increase in activity. It is interesting to observe that the
activity was still enhanced when the loading of Co was reduced with the
loading of Cu and Mn keeping the maximum value. The CH4 selectivies
showed an opposite trend to the activity. The selectivities for the
formation of C2ꢀC4 hydrocarbons remain almost unchanged for all the
catalysts. The selectivity towards CO2 formation on the 15Co4Cu4Mn,
15Co8Cu8Mn, and 8Co8Cu8Mn catalysts were slightly higher than that
of the 15Co catalyst, which can be attributed to the addition of copper
that enhances the water gas shift reaction. The selectivities of C5+
hydrocarbon significantly increases as 4% Cu and 4% Mu were added
into the 15Co catalyst and then decreased to lowest in the sequence of the
15Co8Cu8Mn and 8Co8Cu8Mn catalysts. All catalysts produce alcohols,
among which the 15Co catalyst showed the lowest selectivity to alcohols
but exhibited a minor increase in alcohol selectivity when 4% Cu and 4%
Mn were added. When the content of Cu and Mn were doubled, the
selectivity to alcohols ramped up slowly, however, a significant increase
in alcohol selectivity can be observed when the loading of Co were
reduced to 8% with the content of Cu and Mn keeping doubled. The
fraction of methanol and C6ꢀC14 alcohols decreased while that of C2ꢀC5

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alcohols increased in the sequence of the 15Co, 15Co4Cu4Mn,
15Co8Cu8Mn and 8Co8Cu8Mn catalysts, which indicated that the
addition of Cu and Mn favored to confine the formation of alcohols in the
carbon number range of 2 to 5.
Table 1 Performance of FT reaction over the 15Co catalyst and the CoCuMn/AC catalystsa.
Selectivity ( C% )
Alcohol distribution (C%)
CO conv.
( C% )
Catalyst
Total
b
c
CH4
C2ꢀC4
CO2
C5+
MeOHd
C2ꢀC5OHe
C6ꢀC14OHf
alcohols
15Co
34.6
52.2
56.0
58.3
32.0
14.1
8.9
21.9
22.0
26.1
22.5
0.9
3.8
5.9
5.2
25.1
36.6
34.1
24.5
20.1
23.5
25.0
40.1
10.1
6.6
3.1
2.7
47.1
54.2
68.5
80.0
42.8
39.2
28.4
17.3
15Co4Cu4Mn
15Co8Cu8Mn
8Co8Cu8Mn
7.7
a Reaction conditions: P = 3.0 MPa, GHSV =500 hꢀ1, T = 493 K.
b Hydrocarbons with carbon numbers from 2ꢀ4.
c Hydrocarbons with carbon numbers more than 4.
d Methanol.
e Alcohols with carbon numbers of 2ꢀ5.
f Alcohols with carbon numbers of 6ꢀ14.
15Co4Cu4Mn
15Co
0
5
10
15
20
0
5
10
15
20
Carbon number (n)
Carbon number (n)
8Co8Cu8Mn
15Co8Cu8Mn
0
5
10
15
20
0
5
10
15
20
Carbon number (n)
Carbon number (n)
Fig. 1 The ASF product distribution of the 15Co and the CoCuMn/AC catalysts: (●) Alcohol (■) Paraffin (▼) Olefin.

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The AndersonꢀSchulzꢀFlory (ASF) product distributions for the
formation of alcohols, olefins and paraffins over the 15Co and the
CoCuMn/AC catalysts are presented in Fig.1 (showing data for alcohols
and olefins up to C14 and paraffins up to C18). There are slight deviations
in the case of paraffins with fewer than nine carbons (such as methane
andheptane), olefins with fewer than seven carbons (such as ethylene
and hexene), and alcohols with one or two carbons that we believe may
be ascribed to an acceptable level of experimental error. Generally,
though, the product distributions all generated linear ASF plots. This
indicates a chain growth mechanism by successive addition of a C1
species in the insertion process as previously proposed for the chain
growth in the literature [14]. Table 2 lists the chainꢀgrowth probability
factors (α) for paraffin, olefin and alcohol products. The α values for
alcohols are seen to be decreased while those of paraffins remain almost
constant, whereas those for olefins increased slightly in the sequence of
the 15Co, 15Co4Cu4Mn, 15Co8Cu8Mn, 8Co8Cu8Mn catalysts. This
result again demonstrated that shortꢀchain alcohols are favorably
produced when Cu and Mn are doped into the 15Co catalyst.
Table 2 Chain growth probability factors of alcohols, paraffins, and olefins of the 15Co and the
CoCuMn/AC catalysts.
catalysts
15Co
0.67
0.67
0.59
15Co4Cu4Mn
0.63
15Co8Cu8Mn
0.62
8Co8Cu8Mn
0.60
αalcohols
αparraffins
αolefins
0.71
0.67
0.71
0.59
0.64
0.66

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3.2. TPR results
466
8Co8Cu8Mn
15Co8Cu8Mn
621
530
15Co4Cu4Mn
15Co
400
500
600
700
800
900
1000
Temperature (K)
Fig. 2 TPR profiles of the 15Co and the various CoCuMn/AC catalysts.
TPR experiments were carried out under flowing H2/Ar to
investigate catalyst reducibility for the 15Co and the CoCuMn/AC
catalysts, and the results were displayed in Fig. 2. There were three
reduction peaks at 530, 621 and above 700 K for the 15Co catalyst. The
broad peak above 700 K could be assigned to hydrogenation of surface
oxygenꢀcontaining groups on the AC support in the presence of Co metal
[15,16], while the other two reduction peaks corresponded to a
consecutive reduction of Co3O4 through CoO to Co0 [10,17,18]. Besides
the aboveꢀmentioned peaks, a new peak at ca. 466 K can be clearly
observed on the 15Co4Cu4Mn, 15Co8Cu8Mn and 8Co8Cu8Mn catalysts.

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It is shown by XRD characterization results (see the following section)
that metallic Cu phases were clearly present on the three catalysts after
reduction while it is also reported that copper ions can be easily reduced
to metallic copper at temperature below 523 K [19]. Thus, it is reasonable
to attribute this new peak to the reduction of Cu2+ to metallic Cu. The
peak position of Co3O4 to CoO and CoO to Co reduction was almost the
same on all the catalysts, indicating that the addition of Cu and Mn
speices did not alter the interaction of Co species with the support.
However, the area of CoO to Co reduction peak suddenly reduced when 4%
Cu and 4% Mn components were added into the 15Co catalyst, and then
gradually increased in the sequence of the 15Co4Cu4Mn, 15Co8Cu8Mn
and 8Co8Cu8Mn catalysts. The areas of the CoO to Co reduction peak on
the 15Co4Cu4Mn, 15Co8Cu8Mn and 8Co8Cu8Mn catalysts were much
smaller than that on the 15Co catalysts, which indicated that the three
catalysts had decreased reducibility as compared to the 15Co catalyst. No
peaks corresponding to the reduction of Mn species were indentified
during TPR process. Thus, Mn species did not reduce to a metallic state,
which is in agreement with what the literature reports [20,21].
3.3. XRD results
XRD patterns for the 15Co and the CoCuMn/AC catalysts after
reduction and subsequent passivation were shown in Fig. 3. The 15Co
catalyst showed only one small peak at 2θ = 44.3º, which could be

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attributed to metallic Co (PDF 00ꢀ001ꢀ1259). For the three reduced
CoCuMn/AC catalysts, one can see that besides the metallic Co peak,
new peaks at 2θ = 43.4 and 50.3º appeared, which could be assigned to
metallic Cu (PDF 00ꢀ001ꢀ1241). The intensities of Cu peaks increased
while the intensity of the Co peak decreased in the sequence of 15Co,
15Co4Cu4Mn, 15Co8Cu8Mn and 8Co8Cu8Mn catalysts. This indicated
that large Cu particles and small Co are willing to form according to the
above tendency. Powder Xꢀray diffraction patterns elucidate the
coꢀexistence of distinctive fundamental peaks for both Co and Cu,
indicating that they have not formed an alloy during the reduction process.
This is in consistence with the findings that Co and Cu metals do not
alloy [22ꢀ24]. No apparent peaks ascribed to manganeseꢀcontaining
species can be found, indicating that they were probably quite finely
dispersed on the surface of the activated carbon support.
Cu
Co
8Co8Cu8Mn
15Co8Cu8Mn
15Co4Cu4Mn
15Co
30
35
40
45
50
55
60
2θ
/(o)
Fig. 3. Powder XRD patterns for the 15Co and the CoCuMn/AC catalysts after reduction and
subsequent passivation.

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Cu
Co2C
Co
(4)
(3)
(2)
(1)
30
35
40
45
50
55
60
2θ
/(o)
Fig. 4 XRD patterns of the 15Co and the CoCuMn/AC catalysts after reaction and subsequent
passivation: (1) 15Co; (2) 15Co4Cu4Mn; (3) 15Co8Cu8Mn; (4) 8Co8Cu8Mn.
Fig.4 showed the XRD patterns of the 15Co and the CoCuMn/AC
catalysts after reaction and subsequent passivation. For the used 15Co
catalyst, besides a characteristic peak of metallic Co (at 2θ = 44.3º), new
peaks (at 2θ = 37.0º, 41.2º, 42.5º, 45.7º and 56.6º) belonging to Co2C
(PDF 01ꢀ072ꢀ1369) can be observed, which indicated that Co2C species
were formed during reaction. Similar phenomena were observed over
Co/AC catalysts in the literature [10]. For the used 15Co4Cu4Mn,
15Co8Cu8Mn and 8Co8Cu8Mn catalysts, characteristic peaks of metallic
Co, Co2C and metallic Cu could be identified simultaneously, indicating
their coꢀexistence in the specific sample. The intensities of Co2C peaks
slightly increased in the sequence of the 15Co, 15Co4Cu4Mn and
15Co8Cu8Mn catalysts, but became very weak for the 8Co8Cu8Mn
catalyst. This indicated that the formation of Co2C was promoted on the
15Co4Cu4Mn and 15Co8Cu8Mn catalysts, but inhibited on the

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8Co8Cu8Mn catalyst, as compared with the 15Co catalyst, which
indicated that there were probably not enough Co species available for
the formation of Co2C on the 8Co8Cu8Mn catalyst as compared with the
other three catalysts when Co loadings were reduced. Alternately, there is
probably a dynamic balance between in situ formed Co and Co2C phases
on Co/AC catalysts during reaction, which can be influenced by several
factors such as promoters or Co loadings. In fact, it is reported in the
literatures that many factors are capable of affecting the formation of
Co2C on Co/AC catalysts, such as the presence of promoters [10],
variations in Co particle sizes [11], and/or reaction temperature [25], but a
more thorough analysis is still necessary, which is not the aim of the
present work. The XRD patterns of Cu on the spent 15Co4Cu4Mn,
15Co8Cu8Mn and 8Co8Cu8Mn catalysts were almost the same as those
on the corresponding reduced catalysts aside from very slight increase in
intensities, which indicated that no apparent structure change or particle
aggregation of Cu species occurred. The intensity of Co0 on the used
15Co catalyst was very strong as compared with that on the reduced
catalyst, indicating significant Co aggregation occurred. The spent
15Co4Cu4Mn, 15Co8Cu8Mn and 8Co8Cu8Mn catalysts showed only
weak XRD peaks of metallic Co, which is an indication that small
particles of Co were formed on these catalysts. The intensities of the Co
peaks decreased in the sequence of 15Co4Cu4Mn, 15Co8Cu8Mn and

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8Co8Cu8Mn catalysts, which indicated that the Co particle size decreased
in the same trend. Additionally, the intensities of metallic Co on the spent
15Co4Cu4Mn, 15Co8Cu8Mn and 8Co8Cu8Mn catalysts were even
weaker than those on the corresponding reduced catalysts, which implied
that although some metallic Co particles would be converted to Co2C
species on these AC supported Co catalysts, small Co0 particles can be
maintained during reaction.
3.4 HR-TEM Observations
Fig. 5 HRꢀTEM images of the 15Co and the CoCuMn/AC catalysts after reaction and subsequent
passivation: (a) 15Co; (b) 15Co4Cu4Mn; (c) 15Co8Cu8Mn; (d) 8Co8Cu8Mn.

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We performed HRꢀTEM experiments to obtain structural
information of the 15Co and the CoCuMn/AC catalysts after reaction and
subsequent passivation. The HRꢀTEM image of the 15Co catalyst is
shown in Fig. 5(a). The lattice spacings of 0.202 nm can be interpreted as
the (111) plane of fcc metallic cobalt phases, while that of 0.219 nm can
be assigned to the (111) plane of Co2C phase, suggesting the coꢀexistence
of metallic Co and Co2C species on the used 15Co catalyst which match
well to the XRD results. Fig. 5 (b), (c) and (d) present the HRꢀTEM
micrographs of the used 15Co4Cu4Mn, 15Co8Cu8Mn and 8Co8Cu8Mn
catalysts, respectively. It can be clearly seen from each micrograph that
there are small and nearly spherical nanoꢀparticles neighboring large
nanocrystallines. The small particles with a measured lattice spacing of
0.202 nm can be attributed to the (111) plane of fcc Co, while the large
nanocrystallines with the interplanar spacing of 0.207 nm can be ascribed
to the (111) plane of Cu. It is usually difficult to identify and distinguish
Co from Cu from the interplanar spacings of the (111) plane, because the
values of the d spacings are very similar. However, the results of XRD
characterization of the used 15Co4Cu4Mn, 15Co8Cu8Mn and
8Co8Cu8Mn catalysts are indicative of large Cu crystallites and samll Co
nanoꢀparticles, which reinforced the HRꢀTEM observations. No
crystalline line corresponding to Co2C was observed in these three figures,
indicating the participation of Co2C phases in such structure can be

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excluded.
Now let us attempt to analyze the catalytic properties of the catalysts
with the characterization results in more detail. It has been reported that
Co2C was responsible for the alcohol formation on Co/AC catalysts
[10ꢀ12, 26], and thereby one can note that alcohols were produced on the
15Co catalyst in the presence of in situ formed Co2C species. When the
15Co catalyst were added with 4% Cu and 4% Mn, the Co2C formation
was promoted as compared with the 15Co catalyst, and this effect was
further enhanced when Cu and Mn content continued to increase. One can
also note that the formation of metallic Cu phases followed the same
trend as that of Co2C, but only a gradual slow increase of alcohol
selectivities was obtained. However, the alcohol selectivities were
significantly enhanced when the Co loadings were decreased which
resulted in a substantially reduced formation of Co2C species but a
considerably improved formation of metallic Cu phases on the
8Co8Cu8Mn catalyst. This suggested that although the Co2C and metallic
Cu phases, characteristic sites for alcohol formation, were tunable on
Co/AC catalysts, Co prevailed to steer the products to hydrocarbons.
Thus, it can be speculated that the ratio between alcohol and hydrocarbon
formation sites plays fundamental role in controlling product selectivities.
It is suggested in the literatures that the increase of Co2C formation or
Co2C/Co ratio on Co/AC catalysts could improve alcohol selectivities

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[10,11]. However, we failed to increase alcohol selectivities by utilizing
Co2C active sites in the present case. Fortunately, one can clearly see that
a proper CoꢀCu composition is beneficial to improve alcohol selectivities.
In fact, it has been widely accepted that an appropriate balance of CoꢀCu
composition has to be chosen for the synthesis of alcohols [22]. In
addition, previous reports suggests that only the number of Co0 that
inꢀsitu formed during reaction on Co/AC catalysts are responsible for the
activity (CO conversion) [12,17], which is difficult to determine
quantitatively and remain a task for future work. For this work, one can
see from XRD results that the Co particle size deceased when taking into
consideration the aboveꢀmentioned varying trend of Cu, Mn and Co
content. Thus, we speculated that smaller Co particle probably
contributed to more number of Co0 which would result in increase of
catalyst activity, although the contributions to Co0 number from Co
loading, Co reducibility, and Co2C formation were different for all the
catalysts. It is well known that the formation of alcohols needs a synergy
between closely contacted metallic copper and cobalt species on CoCu
bimetal catalysts [14,27]. Our HRꢀTEM observations concur with the
literatures. More importantly, it can be clearly seen that large Cu particles
are decorated by small Co nanoꢀparticles, which we propose that such
structure was responsible for the preferential formation of C2+ alcohols.
We now turn to compare the catalytic performance of the

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CoCuMn/AC catalysts with that of an AC supported CoCu biꢀmetal
catalyst. This comparison is made in terms of activity, alcohol selectivity,
and alcohol distribution for the 15Co4Cu4Mn catalyst and the 15Co4Cu
catalyst. It can be seen from Table S1 that the performance of activity and
alcohol selectivities of the binary catalyst is much lower than those of the
15Co4Cu4Mn catalyst, and even lower than those of the 15Co catalyst. It
is also noteworthy that methanol formation is much higher than that of
the 15Co4Cu4Mn catalyst, which can be explained if one remembers that
Cu is a methanol producing site. Thus, it is obvious that the addition of
Mn is beneficial for the catalytic performance in higher alcohol synthesis.
XRD characterization of the used 15Co4Cu catalyst revealed the
coꢀexistence of Co, Cu and Co2C phases (see Fig. S1), but it is notable
that the intensities of peaks of Co2C and Co components became the
strongest and the widths of the peaks became the narrowest apparently,
suggesting that severe carbidization and aggregation of Co particles
occurred as compared with other catalysts. The decline of the reaction
rate of Co catalysts is usually attributed to several mechanisms, such as
sintering, catalyst poisoning, active metal reꢀoxidation, metalꢀsupport
compound formation, and carbidization [28]. Our analysis suggested that
the serious carbidization and sintering of Co particles on the 15Co4Cu
catalyst might result in remarkably reduced Co0 number and thereby
significantly decreased reaction rate of the 15Co4Cu catalyst. A careful

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HRꢀTEM examination on the spent 15Co4Cu catalyst revealed that no
structure containing closely contacted Co and Cu species similar to the
cases of the CoCuMn/AC catalysts can be found. On CoꢀCu type
catalysts for alcohol synthesis, the intimate CoꢀCu center is thought to be
the active site for alcohol synthesis, on which Co functioning as CO
dissociative activation and chain propagation site while Cu functioning as
CO molecular activation and insertion site, the synergistic effect between
Co and Cu is crucial to alcohol formation. It is speculated that the
separation of Co and Cu phases might occur on the used 15Co4Cu
catalyst, which would break the synergetic interaction between Co and
Cu active sites [29], making Co easy to suppress alcohol formation. The
interesting result regarding alcohol distribution is that the CoCuMn/AC
catalysts favored the production of C2+ alcohols, with alcohols
preferentially formed in the carbon number range of 2ꢀ5 and methanol
formation severely suppressed. Many factors can be advanced to tune the
shift in the product (hydrocarbon and/or alcohols) distribution of Co
based catalysts for FT reaction, such as promoters [9,12,30], texture
properties of support materials [31ꢀ33], reaction conditions [14,34], etc,
however, studies on the product distribution over Co/AC catalyst systems
are extremely scare. From the literatures [12,17], we know that the
texture properties of the AC support after the introduction of active
components had little impact on catalyst performance. In light of the

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report by Ying et al. [35], who well documented the influence of
operation conditions on the Co/AC catalyst systems, we in fact tested the
catalysts at the optimum reaction conditions in order to obtain the
maximum selectivities towards alcohols. The situation is more
complicated when promoters such Al, Si, and La are added to Co/AC
catalysts [10,12,17]. As for the Mn additive, it is known that Mn usually
provides a dispersion effect of active sites and/or a favorable production
of olefins for CoꢀCu type catalysts [7,36]. With the present work, there is
no doubt that the addition of Mn species led to the formation of closely
contacted Co and Cu phases with the microꢀstructure that smallꢀsized Co
particles were situated on the external surface of large Cu particles, which
we suggested that such structure was responsible for the preferential
formation of C2+ alcohols. For better understanding the mechanism for
the influence of Mn species on the catalytic performance concerning
activity, selectivity as well as product distribution of the CoCuMn/AC
catalysts, further studies would be desirable.
4. Conclusions
A series of CuCoMn/AC catalysts were prepared and tested for FT
reaction. It is found that the CoCuMn/AC catalysts exhibited good
activity and selectivities to alcohols with substantially reduced methanol
production but significant increased formation of C2ꢀC5 alcohols. The
enhanced activity (CO conversion) was attributable to the increase of Co0

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number during reaction, whilst the improved alcohol selectivity was
closely associated with the Cu/Co composition ratio. It is considered that
the decoration of large Cu cluster with small Co nanoꢀparticle was
responsible for the preferential production of C2+ alcohols. Future
optimization of the active sites and catalyst structure are expected to
result in additional improvements in catalytic performance.
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Activated carbon supported CoCu catalysts with structures containing small-sized Co particles
bordering large Cu particles favored formation of C2+ alcohols.