Preparation of promoted molybdenum carbides nanowire for CO hydrogenation

Article abstract of DOI:10.1007/s10562-013-1163-7

K, Mg, and Ca modified nanowire molybdenum carbide have been prepared using an organic-inorganic hybrid method for the production of higher alcohols and hydrocarbons from the hydrogenation of CO. The results revealed that unpromoted molybdenum carbide mainly produced light alkanes, and only few alcohols were obtained. The promoters improved selectivity shift from alkane to alcohols in the sequence: K > Mg > Ca. Additional, the CO conversion was reduced with the addition of K and Mg, with K exhibiting more vigorous effect, but that remarkably increased for Mg. Moreover, the promoters showed different effectiveness on the ability of chain propagation. Potassium mainly enhanced the ability of alcohol chain propagation with higher selectivity to C₂ ⁺OH, but had little effect on that of alkane. While, for magnesium and calcium, they mainly improved the ability of alkanes chain propagation with reducing the formation of methane. Graphical Abstract: A series of novel promoted β-Mo₂C nanowire catalysts was developed by an organic-inorganic hybrid method. It could be seen that promoters evidently had a great effect on the structure and performance of β-Mo₂C nanowire catalyst. Thus the different performance of these catalysts in the CO hydrogenation for hydrocarbon and higher alcohols was related to the synergistic effect of promoters with molybdenum.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-013-1163-7

Catal Lett (2014) 144:567–573
DOI 10.1007/s10562-013-1163-7
Preparation of Promoted Molybdenum Carbides Nanowire
for CO Hydrogenation
•
Changcheng Liu Minggui Lin Dong Jiang
•
•
•
Kegong Fang Yuhan Sun
Received: 5 September 2013 / Accepted: 9 November 2013 / Published online: 4 February 2014
Ó Springer Science+Business Media New York 2014
Abstract K, Mg, and Ca modified nanowire molybde-
num carbide have been prepared using an organic–inor-
ganic hybrid method for the production of higher alcohols
and hydrocarbons from the hydrogenation of CO. The
results revealed that unpromoted molybdenum carbide
mainly produced light alkanes, and only few alcohols were
obtained. The promoters improved selectivity shift from
alkane to alcohols in the sequence: K [ Mg [ Ca. Addi-
tional, the CO conversion was reduced with the addition of
K and Mg, with K exhibiting more vigorous effect, but that
remarkably increased for Mg. Moreover, the promoters
showed different effectiveness on the ability of chain
propagation. Potassium mainly enhanced the ability of
alcohol chain propagation with higher selectivity to
C2?OH, but had little effect on that of alkane. While, for
magnesium and calcium, they mainly improved the ability
of alkanes chain propagation with reducing the formation
of methane.
1 Introduction
Driven by economic development and global warming
effects, the rapidly increasing energy demand posted high
price of oil reserves and encouraged the use of alternative
and clean energies. Among them, the synthesis of synfuels
of hydrocarbons or higher alcohols via carbon monoxide
hydrogenation remains considerable interest [1, 2].
Recently, molybdenum carbide has been widely studied
due to its unique physical and chemical properties similar
to those of noble metals [3–6], and has attracted extensive
attentions in heterogeneous catalysis, such as methane
aromatization [7], methane reforming [8], hydrodesulfuri-
zation [9], hydrodenitrogenation [10], hydrocarbon isom-
erization [11], hydrazine decomposition [12], and
especially CO hydrogenation [13]. It is well-known that
molybdenum carbide was very active in CO hydrogenation
reaction, but the major limitation of Mo2C-based catalysts
in the catalytic reactions is the low selectivity to the desired
products. Up to now, much work has been done to modify
the selectivity towards higher hydrocarbons or higher
alcohols [14–16], whereas the formation of alcohols is
related to the surface stoichiometry and to the extent of
carburization [3]. Actually, the product can shift from
hydrocarbons to higher alcohols when appropriate pro-
moters and reaction conditions are employed. It is claimed
that K exerts both geometric and electronic effects. The
addition of K would physically block the active sites which
are the potential adsorption sites for hydrogen, increase the
electron density of the Mo to increase the surface basicity
of molybdenum carbide, and moderate the strength of the
Mo–CO adsorption that enhances reactivity of CO and
increases alcohol selectivity [16–18]. Alkaline-earth metals
promoted transition metal FT catalysts have been shown to
suppress methane formation and improve C5? selectivity as
Keywords b-Mo2C Á Nanowire Á Promoter Á CO
hydrogenation
C. Liu Á M. Lin Á D. Jiang Á K. Fang
State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, The Chinese Academy of Sciences,
Taiyuan 030001, Shanxi, People’s Republic of China
C. Liu
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
Y. Sun (&)
Low Carbon Conversion Center, Shanghai Advanced Research
Institute, Chinese Academy of Sciences, Shanghai 201203,
People’s Republic of China
e-mail: yhsun@sxicc.ac.cn
123

568
C. Liu et al.
well as FT activity [19, 20]. The similar benefits for Mo2C
from basic metal oxide additives have been proved by Shou
et al. [14] and Vo et al. [15].
then dried at 80 °C for overnight. The nanowires of b-Mo2C
were then obtained from calcining the white precipitate
nanowires in Ar atmosphere (GHSV = 10,000 h-1). The
nanowires precipitate were heated with a ramping rate of
1 °C min-1 to 700 °C and kept at that temperature for 4 h.
Before exposure to air, the sample was quenched to room
temperature and gradually passivated with 2.0 vol% O2/Ar
(GHSV = 2,000 h-1).
So far, the conventional method to prepare molybdenum
carbide is temperature-programmed reduction (TPRe),
which involved the reaction between molybdenum oxides
and a mixture of H2 and carbon-containing gases [13, 21].
However, this method requires complex and strict synthetic
conditions even then incomplete transformation is difficult to
be avoided. Meanwhile, this method is not economical and
green as it needs a high flow gas not only act as carbon-source
but also take away the byproducts (H2O). Besides, the sur-
face of the resultant carbide is usually contaminated by
polymeric carbon from the pyrolysis of excessive gases [22].
Several other methods have been applied to produce
molybdenum carbide, such as pyrolysis of metal precursors
[23], alkalide reduction [24], sonochemical synthesis [25],
carbothermal reduction [26], and thermal carburization
method [22, 27]. Among those methods, the thermal carbu-
rization method has attracted increasing attention in recent
years. Compared with the conventional TPRe method, this
method is simple, and do not require high flow rates of
gaseous hydrocarbons and a slow ramp rate. However, only
bulk or carbon supported carbides can be prepared due to the
ability of the carbon-source support to react with the metal or
metal oxide particles [7]. Recently, Gao et al. [27] have
found that the employment of long-chain fatty amine such as
aniline for organic–inorganic hybrid facilitated the forma-
tion of molybdenum carbide with nanowire feature and
performed well in the probe reaction. Based on the param-
eters of a variety of physical properties such as atomic scale
structures and 1D morphology, the nanowire would express
fascinating properties in catalytic reactions [28].
The M/b-Mo2C (M: K, Mg or Ca) sample was prepared
by physically mixing b-Mo2C nanowire with the corre-
sponding promoter precursor (K2CO3, Mg(OH)2Á4MgCO3Á
6H2O or CaCO3) respectively with M/Mo molar ratio of 0.2.
After grinding, the sample was calcined at 500 °C for 4 h
under Ar.
2.2 Characterization
X-ray diffraction (XRD) characterization was conducted
using a Bruker diffractometer employing Cu Ka radiation
(40 kV and 40 mA). Single-point Brunaeur–Emmett–Teller
(BET) surface area data were acquired using an ASAP-2000
Micromeritics instrument. Scanning electron microscopy
(SEM) images were taken with a Philips XL30 electron
microscope operating at 20 kV. Transmission electron
microscopy (TEM) images were recorded using a JEOL-
2011 microscope operated at 200 kV. X-ray photoelectron
spectroscopy (XPS) experiments were carried out with a
PHI-5300 X photoelectron spectrometry employing a Al-Ka
X-ray source, The binding energy of the samples was cor-
rected with the energy of C1s (284.6 eV) as the reference.
2.3 Catalysts Test
Therefore, the objective of this paper is to investigate
the role of K, Mg and Ca promoters in the physicochemical
properties of nanostructured molybdenum carbide catalysts
and CO hydrogenation metrics.
The CO hydrogenation reactions were carried out using a
stainless steel fixed-bed reactor with 1.0 ml (40–60 mesh)
of catalyst at nominally identical conditions of 340 °C,
7.0 MPa, 4,000 h-1 (GHSV), and H2/CO ratio of 2.0. Both
gaseous and liquid products were analyzed off-line by gas
chromatographs. H2, CO, CH4 and CO2 were determined
by thermal conductivity detector (TCD) with a TDX-101
column. The water and methanol in liquids were also
detected by TCD with a GDX-401 column. The alcohols
were analyzed by flame ionization detector (FID) with a
Porapack-Q column. The carbon balance and mass balance
were 100 5 %.
2 Experimental
2.1 Catalyst Preparation
Nanowires molybdenum carbide was prepared through an
organic–inorganic hybrid method followed by a tempera-
ture-programmed carburization process as reported in Ref.
[16]. In detail, 4.86 g of ammonium heptamolybdate
((NH4)6Mo7O24Á4H2O) and 9.86 g of hexamethylene dia-
mine (1,6-H2N(CH2)6NH2) were dissolved in 90 ml of
distilled water. Afterwards, 2.0 M HNO3 aqueous solution
was dropwise added with magnetic stirring until a white
precipitate appeared (pH 3–4). After reaction at 60 °C for
4 h, the product was filtered and washed with ethanol, and
3 Results and Discussion
3.1 Structure of the Catalysts
The XRD patterns of the samples are shown in Fig. 1. It can
be seen that several sharp and strong peaks located at
123

Preparation of Promoted Molybdenum Carbides Nanowire for CO Hydrogenation
569
2h = 34.4°, 38.0°, 39.4°, 52.1°, 61.5°, 69.6° and 74.6°
belonging to b-Mo2C [21] was definitively detected, proving
b-Mo2C was successfully synthesized by the organic–inor-
ganic method. Moreover, the intensity of peaks assigned to
b-Mo2C became slightly weaker with the addition of pro-
moter. As for the K/Mo2C, no other peaks can be found, that
could be derived from the precursor of K highly dispersed in
the samples and reacted with Mo species to produce new
phases, such as K2Mo2O7, KMo4O6 [22]. No other peaks are
also observed in the sample of Mg/Mo2C, which could be
ascribed to MgO, generated from thermal decomposition of
the precursor of Mg under the high calcination temperature,
highly dispersed in the sample [23]. Additional, no other
substance is produced from MgO and Mo species under the
conditions, as reported by others [19, 20, 24]. As for Ca/
Mo2C, a peak located at 29.5° assigned to CaCO3 emerges,
that is most likely due to CaCO3 could not process decom-
position in the calcination progress [25]. It can be seen that
the peaks of MoO2 at 26.1° and metallic Mo at 40.5° are
observed in all the patterns. This result would be result from
the low content carbon-source in the precursor flee with high
Ar flow and not enough to react with Mo species to produce
b-Mo2C [4].
The morphology of unpromoted and promoted b-Mo2C
samples were studied by SEM (Fig. 2). In Fig. 2a the
obtained b-Mo2C showed uniform 1D morphology of
several micrometers in length and 50–120 nm in width
although some of the nanowires seem to be stacked to
bundles. As for the promoted samples, the addition of
promoter produces a shorter and disordered nanowire, the
surface of which is decorated by some clusters, probably
due to of grinding and sintering.
TEM was used to determine the specific structural fea-
tures of samples (Fig. 3). Although some of the unpro-
moted b-Mo2C nanowires seem to be stacked to bundles in
the SEM image of Fig. 2a, the TEM observation with
higher magnification shows that they are still essentially
20
30
40
50
2θ/(o)
60
70
80
Fig. 1 XRD pattern of samples. a b-Mo2C, b Ca/b-Mo2C, c K/b-
Mo2C and d Mg/b-Mo2C
Fig. 2 SEM images of the catalysts. a b-Mo2C, b Ca/b-Mo2C, c K/b-Mo2C and d Mg/b-Mo2C
123

570
C. Liu et al.
Fig. 3 HRTEM images of
catalysts. a b-Mo2C, b Ca/b-
Mo2C, c K/b-Mo2C and d Mg/
b-Mo2C
individual nanowires. Figure 2a further displays that such
nanowires are composed of nanoparticles (5–10 nm), and
notably there is rarely carbon layer observed on the surface
of Mo2C nanoparticles in nanowires by TEM. Additional,
TEM images further confirm the addition of promoters lead
nanowires becomes disordered and the surface is decorated
by some clusters, and such different change is more evident
for Ca promoted b-Mo2C catalyst. Combined with XRD, it
seems that promoters almost insert into the bulk b-Mo2C,
the interaction between Mo and promoters would have a
great influence on the catalytic performance.
Mo2C (3d5/2 = 228.6 eV) were observed [31], in accord
with the XRD spectra. There was no Mo0 peak observed,
although our XRD result reveals poorly crystallized Mo
phase in the sample (Fig. 1). The Mo3d spectra of the
catalysts were deconvoluted to yield several Mo ions with
different valences (see Table 1). Upon promoted, the Mo3d
slightly shifted to higher B.E., especially K. The ‘‘K–Mo–
C’’ entities would be envisaged in the K adapted sample
[32], and the intensely synergetic interaction between K
atoms and Mo atoms leading the electron transformation
from alkali atoms to Mo atoms. The electronic donation
from alkali atoms to Mo atoms resulted in the increase of
d-orbital occupation of molybdenum which further
increases the intrinsic activity, indicating the additives play
a great role in the chemical environment of Mo.
3.2 Surface Properties
The XPS measurement is applied to investigate the surface
properties of the samples, and the corresponding results of
each sample are given in Fig. 4. Three Mo3d peaks at
228.8, 232.7 and 235.9 eV were observed for the unpro-
moted nanowire b-Mo2C catalyst. For b-Mo2C, the major
molybdenum species, Mo5? (3d5/2 = 231.5 eV) and Mo6?
(3d5/2 = 232.9 eV), are found on its surface. These peaks
were characteristic of oxidized phases, resulting from the
ambient oxidation in passivation step [29, 30]. The species
with low oxidation states, MoO2 (3d5/2 = 229.4 eV) and
For each C1s spectrum, the major peak at about
284.6 eV is assigned to the surface C–C bonds [28, 29, 33],
indicating a large amount of carbon on the surface of cat-
alysts. When promoted by Ca and Mg, it is noted that the
C1s with higher BE are increased in the samples, especially
a peak at 290.4 eV assigned to CO3– group emerged. In all
the samples, the signal for the carbon atoms in the carbide
form was very weak, almost cannot be distinguished
clearly.
123

Preparation of Promoted Molybdenum Carbides Nanowire for CO Hydrogenation
571
Fig. 4 Mo3d, O1s and C1s
284.6
Mo3d
C1s
XPS patterns of catalysts. a b-
Mo2C, b Ca/b-Mo2C, c K/b-
Mo2C, and d Mg/b-Mo2C
233.1
231.9
d
c
d
c
285.5
286.4
228.7
229.7
233.2
232.3
231.2
284.4
285.3
229.0
284.4
284.7
233.4
231.7
b
285.4
286.3
b
a
229.5
228.6
290.4
a
232.9
231.5
228.6
229.4
285.6
242 240 238 236 234 232 230 228 226
Binding Energy/eV
290
288
286
284
282
280
Binding Energy/eV
O1s
530.7
531.9
533.2
d
530.8
c
532.1
531.0
532.3
b
533.5
530.7
531.7
532.8
a
538
536
534
532
530
528
526
Binding Energy/eV
For each O1 s spectrum, the first peak at about 530.7 eV
is ascribed to lattice oxygen in the molybdenum oxides or
oxygen in surface –COO– functional group [34]. When
promoted with Ca and Mg, it is identified that a peak at
about 533.3 eV was appeared, suggesting lot of the pre-
cursor of these adapters on the surface of catalysts.
indicated that they were almost nonporous. Combined
with TEM and SEM images, it could be concluded that
the promoters could not wholly insert into the bulk b-
Mo2C. Additional, the surface ratio of carbon to molyb-
denum was much higher than its stoichiometry. It showed
that amorphous carbon was unavoidable during the prep-
aration procedure, which was consistent with C1s spectra
in XPS.
The surface compositions and BET surface area of the
samples determined by XPS are listed in Table 2. The
surface promoter content was all higher than its nominal
preparation, suggesting an enrichment of promoter on the
surface of catalysts. It could be seen that the BET surface
area of b-Mo2C catalyst is 12.0 m2 g-1, while the pro-
moted samples show lower BET surface areas. This
3.3 Catalytic Performance
The catalytic performance in CO hydrogenation over pro-
moted b-Mo2C catalysts are shown in Tables 3 and 4.
123

572
C. Liu et al.
alkanes, only *4 wt% alcohols obtained. As the main
product in alcohols, methanol was about 90 wt%.
Table 1 Quantitative analysis of Mo3d signals for promoted b-Mo2C
catalysts
Upon addition promoters to molybdenum carbide, it
could be seen that the performance of the catalysts showed
remarkable deviation in the CO hydrogenation reaction.
The potassium promoted catalyst yield the lowest overall
CO conversion of 24.5 % at the base reaction condition of
340 °C and 7.0 MPa, with a H2/CO ratio of 2.0 and GHSV
of 4,000 h-1, yet the product shifted remarkably from
hydrocarbons to alcohols. Additional, from the product
distribution, it could be seen that the addition of potassium
enhanced the ability of chain propagation and facilitated
the C2?OH formation, yet K had little effect on the chain
propagation of hydrocarbon. When modified with magne-
sium, the catalyst showed the highest CO conversion, and
appreciably promoted the alcohols synthesis. It could be
noted that the addition of Mg restrained the formation of
methanol and methane to propagate the chain, especially
the hydrocarbons. Ca promoted catalyst produced CO
conversion similar to unpromoted catalyst, and almost the
same alcohols distribution. But similar to Mg, the addition
of Ca showed certain ability for the formation of C2?H. In
terms of catalytic performance, the more Mo species with
lower valence in the catalysts would favor the CO con-
version [35, 36]. Meanwhile, the more percent of Mo4? and
Mo5? on the surface of b-Mo2C might favor the alcohols
synthesis, whereas the lower valence of Mo (Mo2?and
Mo0) might favor the hydrocarbons synthesis [16, 37]. In
other words, to get a good Mo2C-based catalyst with per-
fect performance in the CO hydrogenation for higher
alcohols or higher hydrocarbons production, there should
be a good balance of Mo3d species in the sample to be
controlled. Overall, the alkali promoter moderates the
strength of the Mo-CO adsorption and then enhances
reactivity and improves product selectivity [14, 16].
Combined with XPS analysis, it could be seen that the
addition of K promoted the valence of Mo to higher
valence. The less species of Mo with lower valence in K/b-
Mo2C reduced the CO conversion, but the more species of
Mo with higher valence promoted the higher alcohols
synthesis. The slight effect of Ca and Mg on Mo3d valence
variation played a little role in promotion higher alcohols
production, but improved the chain growth probability of
alkanes. As a whole, the addition of Ca and Mg suppressed
CH4 formation but improved C2?H selectivity [15]. Com-
plex the physicochemical properties of promoter precur-
sors, such as melting point, solubility and basicity, and
then, it could be concluded that potassium promoted cat-
alyst seemed to be the potential catalyst for higher alcohols
synthesis, and the alkaline-earth promoted catalyst favored
for higher hydrocarbon synthesis.
Catalyst
Assignment
B.E. (eV)
Area (%)
b-Mo2C
Mo6?
Mo5?
Mo4?
Mo2?
Mo6?
Mo5?
Mo4?
Mo2?
232.9
231.5
229.4
228.6
233.4
231.7
229.5
228.6
233.2
232.3
231.3
229.0
233.1
231.9
229.7
228.7
44.0
18.8
15.6
21.6
49.9
19.8
20.2
10.1
56.7
7.7
Ca/b-Mo2C
K/b-Mo2C
Mg/b-Mo2C
Mo6?
Mo6?-Mo5?
Mo4?-Mo5?
Mo2?-Mo4?
Mo6?
Mo5?
Mo4?
23.1
12.5
48.3
17.1
15.7
18.9
Mo2?
Table 2 BET area and surface compositions of promoted b-Mo2C
catalysts
Catalyst
SBET (m2 g-1
)
Surface atom content (mol%)a
XMo
XM
XC
XO
b-Mo2C
Ca/b-Mo2C
K/b-Mo2C
Mg/b-Mo2C
a
8.9
12.0
2.8
12.29
9.83
9.25
5.81
0
67.55
53.54
65.96
63.92
20.16
31.30
20.04
25.40
5.33
4.75
4.87
4.4
Calculated from XPS data
Table 3 Catalytic performances of the promoted b-Mo2C catalysts in
CO hydrogenation
Cat.
CO
(%)
STYROH
[g (ml h)]
STYHC
[g (ml h)]
Selectivity
(wt%)a
ROH
HC
Mo2C
79.41
76.33
24.51
90.53
0.018
0.029
0.060
0.046
0.488
0.525
0.096
0.713
3.64
5.23
96.36
94.77
61.36
93.88
Ca/Mo2C
K/Mo2C
Mg/Mo2C
38.64
6.12
Reaction condition, T = 340 °C, P = 7.0 MPa, n(H2)/n(CO) = 2.0,
GHSV = 4,000 h-1
a
Calculated on a CO2 free basis
Indeed, b-Mo2C with nanowire morphology exhibited high
catalytic activity for CO hydrogenation in high-pressure
reaction. It could be seen that the main products were light
123

Preparation of Promoted Molybdenum Carbides Nanowire for CO Hydrogenation
573
Table 4 Product distribution over the promoted b-Mo2C catalysts
Cat.
Alcohol distribution (wt%)
Hydrocarbon distribution (wt%)
MeOH
EtOH
PrOH
BuOH
C5?OH
CH4
C2H6
C3H8
C4H10
C5?H
Mo2C
90.81
92.62
48.40
85.14
7.29
5.73
1.66
1.25
0.16
0.25
3.04
1.18
0.07
0.15
1.81
0.23
67.78
57.45
62.69
42.25
20.59
27.06
20.62
33.92
6.65
9.76
1.68
2.40
3.29
4.19
3.30
3.33
4.40
4.74
Ca/Mo2C
K/Mo2C
Mg/Mo2C
33.13
9.65
13.63
3.80
9.00
14.89
Reaction condition, T = 340 °C, P = 7.0 MPa, H2/CO = 2.0, GHSV = 4,000 h-1
6. Levy RB, Boudart M (1973) Science 181:547
4 Conclusion
7. Shu Y, Xu Y, Wong S, Wang L, Guo X (1997) J Catal 170:11
8. Darujati A, Thomson W (2005) Appl Catal A 296:139
9. Wang Z-Q, Zhang Z-B, Zhang M-H (2011) Dalton Trans 40:1098
A series of promoted nano-structured molybdenum carbide
catalysts was prepared by an organic–inorganic method
followed by temperature-programmed carburization pro-
cess under flowing Ar. The characterizations such as TEM,
XPS and XRD confirmed the synergetic interaction
between promoters and Mo. Further, the promoters of K,
Ca, and Mg played a special role in the reaction of CO
hydrogenation, in other words, the promoters optimized the
CO conversion and products selectivity. Overall, the
addition of promoters enhanced the alcohols selectivity. In
more specific terms, K remarkably facilitated alcohols
synthesis at the expense of CO conversion, and K enhanced
the ability of chain propagation of b-Mo2C with higher
selectivity to C2?OH. Mg and Ca suppressed CH4 forma-
tion but improved C2?H selectivity. The different perfor-
mance in the reaction was most likely due to the synergistic
effect of promoters with molybdenum and the formation of
new reactive sites in all catalysts.
´
10. Szymanska-Kolasa A, Lewandowski M, Sayag C, Brodzki D,
´
Djega-Mariadassou G (2007) Catal Today 119:35
11. Han J, Duan J, Chen P, Lou H, Zheng X, Hong H (2011) Green
Chem 13:2561
12. Zheng M, Shu Y, Sun J, Zhang T (2007) Catal Lett 121:90
13. Park KY, Seo WK, Lee JS (1991) Catal Lett 11:349
14. Shou H, Davis RJ (2011) J Catal 282:83
15. Vo D-VN, Arcotumapathy V, Abdullah B, Adesina AA (2013)
Catal Today 214:42
16. Zaman S, Smith KJ (2012) Catal Rev 54:41
17. Lee JS, Kim S, Kim YG (1995) Top Catal 2:127
18. Solymosi F, Bugyi L (2000) Catal Lett 66:227
19. Luo M, Davis BH (2003) Appl Catal A 246:171
20. Bao A, Liew K, Li J (2009) J Mol Catal A 304:47
21. Xiao T, Wang H, Da J, Coleman KS, Green MLH (2002) J Catal
211:183
22. Wang HM, Wang XH, Zhang MH, Du XY, Li W, Tao KY (2007)
Chem Mater 19:1801
23. Li XL, Li YD (2004) Chem Eur J 10:433
24. Li Y, Fan Y, Chen Y (2003) J Solid State Chem 170:135
25. Park H, Kim M-H, Hwang YK, Chang J-S, Kwon Y-U (2005)
Chem Lett 34:222
26. Liang C, Ying P, Li C (2004) Chem Mater 14:3148
27. Gao Q, Zhang C, Xie S, Hua W, Zhang Y, Ren N, Xu H, Tang Y
(2009) Chem Mater 21:5560
28. Joshi RK, Schneider JJ (2012) Chem Soc Rev 41:5285
29. Oshikawa K, Nagai M, Omi S (2001) J Phys Chem B 105:9124
30. Delporte P, Meunier F, Pham-Huu C, Vennegues P, Ledoux MJ,
Guille J (1995) Catal Today 23:251
Acknowledgments The authors acknowledge the financial support
from the State Key Foundation Program for Development and
Research of China (Contract No. 2005CB221402), the National High
Technology Research and Development Program of China (No.
2012AA051002) and the projects of National Natural Science Foun-
dation of China (No. 21103217).
31. Clair TPS, Oyama ST, Cox DF, Otani S, Ishizawa Y, Lo RL,
Fukui K-I, Iwasawa Y (1999) Surf Sci 426:187
32. Xiang M, Li D, Li W, Zhong B, Sun Y (2007) Catal Commun
8:503
References
33. Surisetty VR, Dalai AK, Kozinski J (2011) Appl Catal A 393:50
34. Khan N, Dollimore D, Alexander K, Wilburn FW (2001) Ther-
mochim Acta 367–368:321
1. Surisetty VR, Dalai AK, Kozinski J (2011) Appl Catal A 404:1
2. Khodakov AY, Chu W, Fongarland P (2007) Chem Rev
107:1692
35. Saito M, Anderson RB (1980) J Catal 63:438
36. Tatsumi T, Muramatsu A, Tominaga H-O (1986) J Catal 101:553
37. Muramatsu A, Tatsumi T, Tominaga H (1992) J Phys Chem
96:1334
3. Leclercq L, Almazouari A, Dufour M, Leclercq G (1996) In:
Oyama ST (ed) The chemistry of transition metal carbides and
nitrides. Blackie, Glasgow, p 345
4. Jansen SA, Hoffmann R (1988) Surf Sci 197:474
5. Hwu HH, Chen JG (2005) Chem Rev 105:185
123