Preparation of nanostructured molybdenum carbides for CO hydrogenation

Article abstract of DOI:10.1039/c4ra01586j

Novel Mo₂C/C nano/microcomposites were prepared via a facile approach involving the hydrothermal carbonization of a solution of glucose as a carbon precursor in the presence of ammonium heptamolybdate tetrahydrate. The samples were subsequently characterized by X-ray diffraction, X-ray photoelectron spectroscopy, thermal gravimetric analysis, N₂- physisorption, scanning electron microscopy and high-resolution transmission electron microscopy. The effect of the carbonization agents (Ar, CH ₄/H₂ = 0.2, and H₂) was investigated. In particular, the carbonization behaviours and the evolution of Mo species in the catalysts during the carbonization process and the effect on CO hydrogenation for higher alcohol synthesis were extensively studied. During the carbonization step, the catalyst structures experienced an extensive restructuring process, which in turn induced the different performances in the higher alcohol synthesis. Moreover, the nanostructured molybdenum carbides synthesized by this method exhibited great performances in CO hydrogenation for higher alcohol synthesis. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01586j

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Preparation of nanostructured molybdenum
carbides for CO hydrogenation
Cite this: RSC Adv., 2014, 4, 20948
ab
a
a
c
c
Changcheng Liu, Minggui Lin, Kegong Fang, Yan Meng and Yuhan Sun*
Novel Mo
carbonization of a solution of glucose as
heptamolybdate tetrahydrate. The samples were subsequently characterized by X-ray diffraction, X-ray
photoelectron spectroscopy, thermal gravimetric analysis, -physisorption, scanning electron
microscopy and high-resolution transmission electron microscopy. The effect of the carbonization
agents (Ar, CH /H ) was investigated. In particular, the carbonization behaviours and the
¼ 0.2, and H
2
C/C nano/microcomposites were prepared via a facile approach involving the hydrothermal
a
carbon precursor in the presence of ammonium
N
2
4
2
2
evolution of Mo species in the catalysts during the carbonization process and the effect on CO
hydrogenation for higher alcohol synthesis were extensively studied. During the carbonization step, the
catalyst structures experienced an extensive restructuring process, which in turn induced the different
performances in the higher alcohol synthesis. Moreover, the nanostructured molybdenum carbides
synthesized by this method exhibited great performances in CO hydrogenation for higher alcohol synthesis.
Received 24th February 2014
Accepted 12th March 2014
DOI: 10.1039/c4ra01586j
www.rsc.org/advances
carbonization of carbohydrates in the presence of a metal
species using a hydrothermal process under mild conditions.
1
. Introduction
Molybdenum carbide has been extensively studied due to its Furthermore, this is an easy, economic and efficient strategy for
unique physical and chemical properties, including mechanical the controllable preparation of nanostructured carbides, and
hardness, thermal stability, tolerance of carbon deposition and can be used for large-scale manufacture and applications.
1
–3
sulfur, and surface reactivity. It has also been identied as a
promising catalyst that can substitute noble metals in several drolysis–carbonization process using glucose and ammonium
Herein, we have designed a one-pot hydrothermal cohy-
4–9
heterogeneous reactions,
especially CO hydrogenation for molybdate as the starting materials and successfully fabricated
carbonaceous spheres embedded with highly dispersed
1
0,11
obtaining higher alcohols.
So far, a variety of methods have been investigated for the molybdenum carbide nanoparticles. The as-prepared materials
synthesis of b-Mo
2
C nanoparticles, such as a sonochemical were characterized by X-ray diffraction (XRD), X-ray photoelec-
12
13
method,
microwave-assisted method,
2
organic–inorganic tron spectroscopy (XPS), N -physisorption, thermal gravimetric
14
15
hybrid method, hydrothermal method, and a carbothermal analysis (TGA), scanning electron microscopy (SEM) and
16
hydrogenation reduction method. Recently, the one-pot transmission electron microscopy (TEM). Based on the data, the
saccharide hydrothermal-carburization process has been effect of the carburization agent upon the catalytic properties
extensively studied for the preparation of metal–carbon mate- was evaluated by CO hydrogenation.
17–21
rials .
Due to the reducibility of saccharides, it is possible to
achieve the preparation of homogeneously dispersed metal
nanoparticles within certain porous solid matrixes or perme- 2. Experimental
able shells, where active nanoparticles are largely isolated and
2.1. Synthesis
their agglomeration under the reaction conditions can be pre-
vented. The porous materials herein act as a physical barrier but
still allow reactants and products to travel freely between the
bulk reaction media and catalyst surface. Generally, carbon-
dispersed nanoparticles are prepared through the direct
Commercially available solvents and reagents were used as
received. Highly dispersed molybdenum carbide nanoparticles
embedded with carbon were prepared via a facile hydrothermal
method followed by carbonization in a certain atmosphere.
19
Typically, 2.88
NH Mo 24$4H
aqueous H and 80 mL distilled water, followed by the
g
MoO
3
[obtained from calcining
(
4
)
6
7
O
2
O (AHM)] was dissolved in 22 mL 30 wt%
2 2
O
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
addition of 19.82 g D-glucose (GL). Aerwards, the solution was
transferred to a Teon-lined stainless steel autoclave (150 mL in
volume) and hydrothermally treated at 453 K for 6 h. Aer
cooling, the dark product precipitate was ltrated, rinsed and
Academy of Sciences, Taiyuan 030001, Shanxi, P.R. China
b
University of Chinese Academy of Sciences, Beijing, 100049, P.R. China
c
Low Carbon Conversion Center, Shanghai Advanced Research Institute, Chinese
Academy of Sciences, Shanghai, 201203, P.R. China. E-mail: sunyh@sari.ac.cn
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dried. This as-synthesized sample was then loaded into a tube
furnace and heated at 973 K for 4 h at a heating rate of 2 K
ꢀ
1
min in streams of Ar, 20 vol% CH /H , and H (GHSV ¼
4
2
2
ꢀ
1
6
000 h ), respectively. Finally, the samples were quenched to
ambient temperature and passivated with 2.0 vol% O /Ar (GHSV
1000 h ) before exposure to air. The carbonized samples are
denoted as Mo–Ar, Mo–CH, and Mo–H, corresponding to
treatment atmosphere of Ar, 20 vol% CH /H , and H , respec-
tively. The sample before carbonization is denoted Mo–Or.
2
ꢀ
1
¼
4
2
2
2.2. Characterization
The bulk structure of the samples was measured by XRD (Rigaku
˚
D/max-RA, Cu Ka radiation, l ¼ 1.542 A). The dimensions and
morphology of the samples were determined using SEM (Hitachi
High-Technologies Co. Ltd. 2.0 kV) and TEM (JEM-2010 FEF,
2
00 kV). The specic surface area and pore volume of the samples Fig. 1 SEM images of (a) Mo–Or, (b) Mo–Ar, (c) Mo–CH, and (d) Mo–H.
were detected using N2 adsorption–desorption isotherms
Micromeritics ASAP-2000). Before analysis, the samples were
(
degassed at 393 K for 12 h. The thermal behavior of the samples
was monitored using TGA (Setaram, Setsys Evolution, 16/18).
During the experiment, the temperature was raised from room
and b, nanostructured molybdenum oxide embedded with
carbon was readily formed aer hydrothermal reaction but even
prior to the carbonization treatment. The nanostructured
molybdenum oxide particles were nely dispersed by the carbon
matrix. Additional, it could be clearly seen from Fig. 2a and b
that the size of the molybdenum oxide nanocrystals is of high
degree of uniformity, of about 4 nm (Table 1). The lattice
spacings of 0.34 nm and 0.24 nm could be assigned to the
ꢀ
1
temperature to 973 K at a heating rate of 10 K min in Ar, H
0 vol% CH /H . Ar was used as the carrier gas with a ow rate
nonuple to that of H or 20 vol% CH /H . The content of carbon
2
or
2
4
2
2
4
2
in the samples was measured with a Carlo Erba 1106 EL Micro-
analyzer. Surface compositions of the samples were investigated
with XPS (PHI-5300 X, Perkin-Elmer Physical Electronics Co.)
using an Al Ka primary radiation. All binding energies were
referenced to C1s of 284.6 eV.
(ꢀ111) and (ꢀ211) planes of the MoO
2
phase, which shall be
further conrmed by XRD analysis. Aer carbonization, there is
no signicant change in the particle size of the encapsulated
molybdenum species nanocrystals (Fig. 2c–h), elucidating the
effectiveness of the carbon matrix in the Mo–Or to restrain the
crystal growth during the carbonization treatment. The mean
particle size of the Mo species synthesized under different
carburization conditions are list in Table 1.
2.3. CO hydrogenation for higher alcohols synthesis
1.0 mL sample (60–80 mesh) was equivalently diluted with
quartz granules and evaluated in a pressurized xed-bed reactor
ꢀ1
at 553 K, 7.0 MPa, 4000 h with a H
any pretreatment. Effluent gases were analyzed using a Carbo-
sieve-packed column with TCD and a Al column with a ame
2
/CO ratio of 2 : 1 without
However, it could be seen that the mean particle size was still
affected by the carbonization agents, which is 5 nm for Mo–Ar, 6
nm for Mo–CH, and 11 nm for Mo–H. Moreover, the crystal
phase of the dispersed molybdenum oxide changed aer
carbonization as expected, the lattice spacings of 0.23 nm, 0.17
nm and 0.26 nm could be assigned to (101), (100), and (102)
2 3
O
ionization detector (FID). A Propake-Q column with TCD and
FID was applied to the analysis of water, methanol and other
alcohol products in the liquids.
2
planes of Mo C (Fig. 2d, f and h), as will be further discussed
together with the XRD analysis below (Fig. 3).
3. Results and discussion
3.1. Morphologies
Besides, adventitious carbon is clearly observed in the

gures and is veried by the element content analysis (Table 1),
In this method, highly dispersed molybdenum carbide nano-
particles embedded with carbon have been prepared in a large
quantity with inexpensive ammonium molybdate and glucose
as precursors. Fig. 1a gives the general morphology and struc-
ture of the sample prepared at 453 K for 6 h under hydrothermal
conditions. As can be seen, the sample shows a porous structure
comprised of small particles. Aer carbonization at 973 K for 4
h, there is almost no difference in the morphologies of these
samples, while more tiny particles appear on the surface, they
are probably due to the aggregation of the Mo species and
gasication of carbon at high temperature.
which is the product obtained from hydrothermal carboniza-
22
tion of D-glucose. Thus, it could be postulated that the unique
combination of nano-sized Mo species particles and adventi-
tious carbon might provide much more active sites than
conventional molybdenum carbides. This approach is expected
to offer an alternative opportunity for designing carbon sup-
ported nano-sized molybdenum carbide for CO hydrogenation
23
due to some potential advantages of the carbon support.
3.2. Phase and textural structure
In order to investigate the microstructure features, the The crystallographic structures of the samples was character-
samples were further characterized by TEM. Shown in Fig. 2a ized by XRD. From the XRD pattern of Mo–Or (Fig. 3a), the
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Fig. 3 XRD patterns of (a) Mo–Or, (b) Mo–Ar, (c) Mo–CH, and (d) Mo–H.
Fig. 4
2
N adsorption–desorption isotherms (at 77 K) of (a) Mo–Or, (b)
Mo–Ar, (c) Mo–CH, and (d) Mo–H.
ꢁ
ꢁ
poorly crystalline phase is assigned to MoO
4
2
(2q ¼ 26.2 , 37.0 ,
ꢁ ꢁ ꢁ ꢁ ꢁ
1.2 , 53.0 , 53.5 , 60.2 and 66.6 , JCPDS no. 32-0671), which
agrees with the HRTEM in Fig. 2b. No additional peaks are
found, conrming that the composite material corresponds to
MoO embedded by carbon. In addition, the carbonaceous
2
phase in these samples is amorphous, since no graphitic carbon
peaks are observed. Aer carbonization in an Ar atmosphere,
the major diffraction peaks of Mo–Ar are still assigned to MoO
The peaks assigned to the peaks of the b-Mo C (JCPDS no. 35-
787) crystalline phase are poor, indicating that not much
MoO has been carbonized to b-Mo C in Ar under the condi-
tions of 973 K for 4 h. These results are conrmed by the
2
.
2
Fig. 2 TEM images of (a and b) Mo–Or, (c and d) Mo–Ar, (e and f) Mo–
CH, and (g, h) Mo–H.
0
2
2
e
Table 1 Textural parameters of the samples
b
Surface atom content (mol%)
a
2
ꢀ1
3
ꢀ1
c
d
Sample
Carbon content (wt%)
C
Mo
O
A
BET (m g
)
TPV (cm g
)
d (nm)
Dc (nm)
Di
Mo–Or
Mo–Ar
Mo–CH
Mo–H
42.73
39.80
35.02
5.28
68.84
80.75
78.55
48.78
2.39
28.77
14.66
11.20
32.16
38.8
305.6
153.2
20.6
0.27
0.48
0.36
0.06
25.9
6.28
12.54
9.33
<4
<4
<4
8
4
5
6
4.59
10.25
19.06
11
a
b
c
Measured with Carlo Erba 1106 EL Microanalyzer. Calculated from XPS data (Mo3d, C1s and O1s). Crystalline size, obtained by Scherrer
d
e
equation: Dc ¼ 0.89l/(b cos q). Mean particle size, observed from HRTEM.
A
BET ¼ BET surface area, TPV ¼ total pore volume, d ¼ average
pore diameter.
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Table 2 Quantitative analysis of Mo3d signals for synthesized samples
Sample
Assignment
B.E./eV
Area/%
6
+
+
Mo–Or
Mo
234.6
232.4
230.8
233.1
231.8
229.9
233.0
230.6
229.5
228.6
233.3
232.1
229.7
228.9
227.9
43.7
40.3
16.0
53.7
23.5
22.8
28.2
13.6
28.4
29.8
24.6
27.2
23.5
20.6
4.1
6
5+
4+
Mo –Mo
5
+
Mo –Mo
6
5
4
6
5
+
+
+
+
+
Mo–Ar
Mo
Mo
Mo
Mo
Mo–CH
4+
Mo –Mo
4
2
6
5
4
2
0
+
+
+
+
+
+
Mo
Mo
Mo
Mo
Mo
Mo
Mo
Fig. 5 The pore size distribution patterns of (a) Mo–Or, (b) Mo–Ar, (c) Mo–H
Mo–CH, (d) Mo–H.
a
Table 3 The performance of catalysts in CO hydrogenation
b
Selectivity
(wt%)
ꢀ1
ꢀ1
Yield (g mL
h )
CO conversion
(C%)
Cat.
Alcohols Hydrocarbon ROH CH
n
Mo–Ar
Mo–CH
Mo–H
12.19
66.55
92.06
0.026
0.038
0.026
0.004
0.069
0.405
0.676
0.331
27.06 72.94
8.67 91.33
3.71 96.29
1.12 98.88
MoC-TPRe 55.79
a
Reaction conditions: H
2
/CO ¼ 2.0, T ¼ 553 K, P ¼ 7.0 MPa, GHSV ¼
ꢀ
1 b
4
000 h
.
Calculated on a CO free basis.
2
HRTEM image in Fig. 2d, the largest lattice spacing is 0.34 nm
and belongs to MoO . As for the sample of MoC–H, the crys-
tallinity of the b-Mo C phase is signicantly improved with
2
2
ꢁ
ꢁ
ꢁ
ꢁ
major diffraction peaks such as 2q ¼ 34.4 , 38.0 , 39.4 , 52.1 ,
ꢁ
ꢁ
ꢁ
6
1.5 , 69.6 , and 74.6 appearing and being enhanced. In
addition, the peak assigned to MoO is too poor to be observed,
which is veried by Fig. 3c. While for the sample of Mo–CH
Fig. 3d), the poorly crystalline peaks could not be distin-
guished, with no peaks even being assigned to MoO , indicating
that MoO in Mo–Or changed aer carbonization in 20 vol%
CH /H . Nevertheless, in Fig. 2f the lattice spacings of 0.24 nm
2
Fig. 6 The TGA and DTG curves of (a) MoC–Ar, (b) MoC–CH
4
, and (c)
(
MoC–H.
2
2
4
2
and 0.23 nm could be assigned to (ꢀ211) plane of MoO and
2
(
101) plane of Mo C. Therefore, these peaks could be assigned
2
2 2 2
to b-Mo C, though not all MoO was transformed to Mo C
under these conditions. Above all, it could be concluded that
different atmospheres led to different carbonizations and
crystallinity degrees of the molybdenum carbide. With refer-
4
24
ence to the work of the Wang and Wang groups, these results
could be explained by the fact that the higher reactivity of H2
benets the reduction of MoO and the formation of b-Mo C at
2
2
lower temperatures.
The N adsorption–desorption isotherms for the samples are
2
displayed in Fig. 4. The isotherms of all samples are of type IV
with the hysteresis loops at relative pressure in the range of 0.4–
1.0, conrming the mesoporous network. The corresponding
Fig. 7 Mo3d and C1s XPS spectra of (a) Mo–Ar, (b) Mo–CH, (c) Mo–H.
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a
Table 4 Alcohols and hydrocarbon distributions over the catalysts
Alcohol distribution (wt%)
Hydrocarbon distribution (wt%)
CH
+
5
+
Cat.
MeOH
EtOH
PrOH
BuOH
C
OH
4
2
C H
6
C
3
H
8
C
4
H
10
C
5
H
Mo–Ar
Mo–CH
Mo–H
72.24
72.58
87.04
83.89
20.34
17.43
7.65
6.10
7.89
3.28
4.06
1.32
1.99
1.14
1.54
0.00
0.12
0.88
0.00
47.17
50.36
41.52
49.74
25.84
25.73
27.75
23.19
16.28
15.20
17.72
15.87
6.08
5.48
6.38
6.04
4.63
3.23
6.63
5.16
MoC-TPRe
10.51
a
/CO ¼ 2.0, T ¼ 553 K, P ¼ 7.0 MPa, GHSV ¼ 4000 hꢀ1
Reaction conditions: H
2
.
25
pore size distribution calculated using the BJH method are of Mo–H and Mo–CH in the XRD spectra(Fig. 3). Moreover, the
shown in Fig. 5. These curves displayed a different pore distri- peaks at about 1000 K could be considered as the result of the
bution center for each sample. For Mo–Or, two peaks centered carbonization of MoO to Mo C. As for the TGA of pure Ar, the
2
2
at 15.1 nm and 32.2 nm appear. Aer carbonization, the overall curve exhibited a gradual and unpronounced weight loss in the
pore distribution center trends smaller. For Mo–CH, the pore range of 450 and 1000 K, which could be attributed to the partial
distribution is centered at 11.3 nm, while for Mo–Ar, two peaks elimination of organic volatiles from the carbonaceous matrix
centered at 3.1 nm and 15.4 nm appear, and for the sample of and the transformation of MoO to MoO . For Ar, the obvious
3 2
Mo–H, the peak centers at about 6.9 nm. This probably results weight-loss appeared at about 1050 K, which is much higher
in the different amounts of carbon le in the samples which than the others. The different temperature of the samples at
provides the porosity.
which the carbonization occurred implies that the presence of
The BET surface area (ABET), total pore volume (TPV) and H apparently decreases the reduction temperature, which is
2
average pore diameter (d) of the samples are presented in Table consistent with the XRD results and other data.
1
. The sample of Mo–Or obtained from hydrothermal treatment
2
ꢀ1
is porous, and the BET surface area is 38.8 m
g . Aer
carbonization, the BET surface area obviously increased, except 3.3. Surface chemistry
for Mo–H. As tabulated in Table 1, Mo–H showed the lowest
The XPS measurement was conducted to investigate the surface
chemistry of the carbonized samples. The Mo3d and C1s
spectra with tting curves are illustrated in Fig. 7. The surface
compositions calculated from the XPS spectra for all samples
are shown in Table 1 and the quantitative analysis of Mo3d
signals for the synthesized samples are shown in Table 2. In
Fig. 7, three peaks located at 226.0, 228.9, and 232.5 eV in the
Mo3d spectra are observed for the samples, except Mo–Or. For
the Mo–Or sample, the major molybdenum species on its
2
ꢀ1
surface area (ABET ¼ 20.6 m g ) and the smallest total pore
3
ꢀ1
volume (TPV ¼ 0.06 cm g ) compared to the others. Mo–Ar
2
ꢀ1
showed the highest surface area (A
¼ 305.6 m
g
) and
BET
3
ꢀ1
largest total pore volume (TPV ¼ 0.48 cm g ), while for Mo–
2
ꢀ1
CH, the surface area is 153.2 m g , and total pore volume is
3
ꢀ1
0
.36 cm g . Combined with elemental analysis and TEM
images, the differences of the texture can be derived from the
carbon materials in the network of samples.
To gain a better understanding of the carbonization process,
thermogravimetric analysis was further employed in this study.
As shown in Fig. 6, the weight loss below 450 K is generally
attributed to the evaporation of adsorbed water in all the
6+
6+
5+
surface are Mo (3d5/2 ¼ 234.6 eV) and Mo –Mo (3d5/2
¼
5+
4+
2
32.4 eV), yet the content of Mo –Mo (3d ¼ 230.8 eV) is low.
5
/2
However, our XRD results reveal only MoO phase in the sample
2
(
Fig. 3a). This conict could be explained by the surface of the
hydrothermally synthesized MoO nanocrystals with low crys-
tallinity being easily oxidized to higher states during the XPS
19
2 4 2
cases. When the sample of Mo–Or carbonized in H or CH /H ,
2
the TG curve exhibited a main weight loss between 450 and 1000
K, which could be attributed to the decomposition of the
carbonaceous material and the reduction–carbonization of
molybdenum oxide. The decomposition of the carbonaceous
material would result in the generation of the porosity, ensuring
better accessibility of the encapsulated molybdenum carbide to
the reaction. H2 would prefer to lead the carbon-source of
precursors to ee in the carbonization process, evidenced by the
fact that a brown tarry substance is present at the end of quartz
19,27
sample preparation and handling.
Aer carbonization, the
overall peaks of the Mo3d spectra move to lower states and the
molybdenum species with low oxidation states become more
observable, which coincides with the XRD analysis. For the Mo–
6+
Ar sample, the major molybdenum species, Mo (3d5/2 ¼ 233.1
19
5+
4+
eV), Mo (3d5/2 ¼ 231.8 eV) and Mo (3d5/2 ¼ 229.9 eV), are
found on its surface. However, no peak belonging to carbidic
molybdenum is observed, although our XRD and HRTEM
tube aer the carbonization process. In addition, CH
as a carbon-donor at higher temperatures, for the sample
4
could act
results reveal a poorly crystallized b-Mo
2
C phase in this sample
(Fig. 3a and 2d). It could be attributed to the ambient oxidation
26
carbonized in CH
the samples. On the other hand, the weight loss in the range of
00–700 K could be ascribed to the reduction of MoO to MoO ,
4 2
/H , as was veried by the carbon content in
2
which turns the surface b-Mo C nanocrystals to molybdenum
19,27
oxides.
For the Mo–CH sample, the molybdenum species
6
3
2
with lower oxidation states, Mo C (3d ¼ 228.6 eV) and MoO2
2
5/2
the presence of MoO could be from the oxidation of MoO and
3
2
(
3d5/2 ¼ 229.5 eV), become more observable (Table 2), which
Mo
2
C, though no peaks were assigned to MoO
3
for the samples
could be attributed to the carbonization in CH /H improving
4
2
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the crystallinity. Nonetheless, the XRD analysis reveals that only concluded that the CO conversion activity is in good accordance
poorly crystallized b-Mo C is detected in the sample. Again, the with the degree of carbonization. In addition, the textural
2
6
+
5+
4+
Mo (3d5/2 ¼ 233.0 eV) and Mo –Mo components (3d
¼
properties of these materials could also play an important role,
30.6 eV) observed herein could be attributed similarly to the and the highly porosity of carbon materials would allow the
ambient oxidation which turns the surface MoO and b-Mo reactants to reach the nanocrystals of the active molybdenum
nanocrystals to a-MoO and H MoO surface species, respec- species. As a result, the higher CO conversion of MoC–H and
While, for the sample of Mo–H, the oxidation states of MoC–CH compared to MoC-TPRe could be ascribed to the
5/2
2
2
2
C
3
x
3
19,27
tively.
4
the molybdenum species are further lower than the others, even mesoporous structure and high surface area of the synthesized
0
Mo (3d5/2 ¼ 227.9 eV) appeared in the sample. Clearly, a higher nanostructured catalysts, as the MoC-TPRe synthesized here is
2
ꢀ1
reduction degree of the Mo species in the Mo–H sample is nonporous and the BET surface area is only 5 m g . Compared
achieved.
with Mo–CH, the lower amount of carbon in the Mo–H sample
The Cls spectrum shows the presence of four overlapping would favor more molybdenum species to react in the reaction
XPS peaks. For Mo–Or, the major peaks at 287.5 and 290.1 eV and show higher activity. On the other hand, the selectivities
are attributed to surface carbonyl or quinone groups (>C]O) towards the products exhibit different trends. It is known that
and carboxylic groups, esters, or lactones (–COO–), probably the formation of alcohols is related to the surface stoichiometry
2
8–30
due to the carbon materials produced from glucose.
Aer and to the extent of the carbonization; whenever the level of
1
carbonization, these peaks with higher BEs signicantly carbonization is too high only hydrocarbons are formed.
decreased, and the major peak at 284.6 eV is assigned to the Combined with the XRD results, it is clearly seen that the trend
surface C–C bonds belonging to amorphous carbon, as reported of selectivity towards alcohols reverses with the degree of
28–30
by several groups.
The other peaks located at around carburization of the samples. As seen in Table 3, the nano-
285.5 and 286.7 eV are attributed to the hydroxyl groups or structured catalysts give better selectivity towards alcohols,
ethers (C–OH) and C–O–C, respectively. The signal for the indicating the advantage of this method. As can be appreciated,
carbon atoms in the carbide form is very weak and almost Mo–Ar gives the highest selectivity towards alcohols (27.1%)
cannot be distinguished clearly, even though in Mo3d spectrum though it performs with the lowest activity. Moreover, in the
the peak belonging to carbidic molybdenum is observed. The absence of MoC-TPRe, Mo–H shows the lowest selectivity
surface compositions calculated from the XPS spectra are towards alcohols (only 3.7%) even though it has the highest
shown in Table 1, it could be expected that the various amounts activity. As shown in Table 3 and 4, it can be seen that the major
of carbon on the surface will result in different catalytic products over these catalysts are C –C hydrocarbons, the yield
1
4
performances in the CO hydrogenation reaction.
of alcohols in the products is much lower than the amount of
hydrocarbons for all the catalysts, and methanol is the main
alcohol product, which is particularly characteristic of the
4
. Catalytic performance
11
action of b-Mo
Finally, to test the catalytic properties of the nanostructured b- XRD results, it could also be seen that the trend of producing
Mo
C, a preliminary study on the CO hydrogenation reaction for methanol parallels with the carbonization degree of the cata-
obtaining higher alcohols using the as-prepared materials was lysts, but the trend of methane is reversed. As for Mo C, it is
carried out. For comparison, the b-Mo
C of the bulky counter- proposed to be a dual-site catalyst involving two types of
part obtained by the TPRe method (marked as MoC-TPRe) was molybdenum species, namely a low valence molybdenum
2
C in CO hydrogenation. Combined with the
2
2
2
34
3
1,32
I
II
also applied to the reaction.
The detailed reaction results are (Mo , I ¼ 0–2) and a high valence molybdenum (Mo , II ¼ 4–5)
I
summarized in Table 3 and 4. Generally, the performance of species. The Mo species is the site for CO dissociation, hydro-
nanostructured molybdenum carbides synthesized by this genation and chain propagation to form surface alkyl species,
II
hydrothermal carbonization method is better than that of the while the Mo species is the site for molecularly adsorbed CO
Mo
2
C obtained by TPRe method in the CO hydrogenation insertion and alcohol formation. Together with XPS and XRD,
reaction. Moreover, the carbonization gases led to catalysts with it could be noticed that the higher amount of Mo species and
different structural and surface properties, with the differences lower amount of Mo in the Mo–H sample, with the higher
32
I
II
specically manifesting in the CO hydrogenation performance carbonization degree, favors the chain propagation of hydro-
of the catalysts. As shown in Table 3, the CO conversion of the carbons and suppresses the chain propagation of alcohols,
catalysts varied across a wide range, for MoC–Ar it is only 12.19 leading to more methanol and less methane in the catalyst.
C%, while it is 92.06 C% for MoC–H, 66.55 C% for MoC–CH
4
,
2
Therefore, to realize a desired Mo C catalyst for higher alcohol
and 55.79 C% for MoC-TPRe. As can be appreciated, transition synthesis it is necessary to precisely control the carbonization
metal carbides have been shown to possess better CO hydro- progress.
genation activity than the corresponding oxides, and the higher
1
level of carbonization, the higher the activity of catalyst.
5
. Conclusions
Furthermore, the activity is closely related to the Mo species
1,33,34
with a lower valence being on the surface of the catalysts.
Moreover, the XPS analysis results (Fig. 7 and Table 2) show that synthesized by a facile hydrothermal method followed by a
the amount of carbon follows the sequence: MoC–H > MoC–CH temperature-programmed carbonization process under
MoC–Ar. Combined with the XRD patterns (Fig. 3), it could be different atmospheres. The subnanometer contact between the
Highly dispersed b-Mo C nanoparticles have been successfully
2
4
>
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RSC Advances
Paper
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Acknowledgements
The authors acknowledge the nancial support from the State
Key Foundation Program for Development and Research of
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