Low-temperature water-gas shift reaction over cobalt-molybdenum carbide catalyst

Article abstract of DOI:10.1016/j.jcat.2006.01.003

The water-gas shift (WGS) reaction over a series of cobalt-molybdenum carbide catalysts was studied at 453 K. The cobalt-molybdenum (Co _xMo_1-x) oxides were prepared using a mixture of aqueous solutions of cobalt nitrate and ammonium heptamolybdate and carburized by a temperature-programmed reaction with a 20% CH₄/H₂ mixture. The catalysts were characterized using X-ray diffraction, CO adsorption, temperature-programmed carburization (TPC) with a 20% CH₄/H ₂ mixture, X-ray photoelectron spectroscopy, and transmission electronic microscopy. The 873 K-carburized Co_0.5Mo_0.5 was found to be the most active among the Co-Mo carbide catalysts; it was more active than a commercial CuZn at 5 min but less active than CuZn at 300 min. The TPC experiment determined the reduction and/or carburization of the Mo and CoMo oxides into the carbides through the oxycarbides. The TPC of the Co _0.5Mo_0.5 oxide showed that the first peak of H ₂O at 747 K was due to the reduction and/or carburization of MoO ₃ to MoO₂ and Mo oxycarbide and of CoMoO₄ to the Co-Mo oxycarbide, and the second peak at 893 K was due to the carburization of the Co-Mo oxycarbide to β-Mo₂C with Co metal and amorphous Co-Mo carbide. In addition, the Co_0.5Mo_0.5 carbide catalyst contained β-Mo₂C, Co metal, and the amorphous Co-Mo oxycarbide (the composition of CoMoC_6.2O_1.4), which grew into crystallites of Co₃Mo₃C after being heated in H ₂. The amorphous Co-Mo oxycarbide is responsible for the WGS reaction.

Full text of DOI:10.1016/j.jcat.2006.01.003

Journal of Catalysis 238 (2006) 489–496
www.elsevier.com/locate/jcat
Low-temperature water–gas shift reaction
over cobalt–molybdenum carbide catalyst
Masatoshi Nagai ∗, Kenji Matsuda
Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24 Nakamachi, Koganei, Tokyo 184-8588, Japan
Received 5 August 2005; revised 19 December 2005; accepted 3 January 2006
Abstract
The water–gas shift (WGS) reaction over a series of cobalt-molybdenum carbide catalysts was studied at 453 K. The cobalt-molybdenum
(Co Mo
) oxides were prepared using a mixture of aqueous solutions of cobalt nitrate and ammonium heptamolybdate and carburized by
x
1−x
a temperature-programmed reaction with a 20% CH /H mixture. The catalysts were characterized using X-ray diffraction, CO adsorption,
4
2
temperature-programmed carburization (TPC) with a 20% CH /H mixture, X-ray photoelectron spectroscopy, and transmission electronic mi-
4
2
croscopy. The 873 K-carburized Co Mo was found to be the most active among the Co–Mo carbide catalysts; it was more active than a
0.5
0.5
commercial CuZn at 5 min but less active than CuZn at 300 min. The TPC experiment determined the reduction and/or carburization of the Mo
and CoMo oxides into the carbides through the oxycarbides. The TPC of the Co Mo oxide showed that the first peak of H O at 747 K was
0.5
0.5
2
due to the reduction and/or carburization of MoO to MoO and Mo oxycarbide and of CoMoO to the Co–Mo oxycarbide, and the second
3
2
4
peak at 893 K was due to the carburization of the Co–Mo oxycarbide to β-Mo C with Co metal and amorphous Co–Mo carbide. In addition, the
2
Co Mo carbide catalyst contained β-Mo C, Co metal, and the amorphous Co–Mo oxycarbide (the composition of CoMoC O ), which
0.5
0.5
2
6.2 1.4
grew into crystallites of Co Mo C after being heated in H . The amorphous Co–Mo oxycarbide is responsible for the WGS reaction.
3
3
2
© 2006 Elsevier Inc. All rights reserved.
Keywords: Co–Mo; Carbide; Water–gas shift; XRD; TPC
1. Introduction
MoS2/Al2O3 [15] and Ni–MoS/TiO2 [12] catalysts exhibit high
activity. Recently, transition metal carbides have been found to
be useful in catalyzing this reaction. For the Mo carbides, Patt
et al. reported high WGS activity with durability for the Mo
carbide, which had an active site density >25% greater than
that of the Cu–Zn catalyst [16]. Moon and Ryu also reported
that the Mo carbide showed higher activity and stability for the
WGS reaction than the commercial low-temperature CuZn cat-
alyst [17]. Thus, molybdenum-containing carbides could have
the bright prospect of developing into alternative catalysts for
replacing the CuZn catalysts in the WGS reaction under the un-
stable oxidized operating conditions.
Despite these prospects, however, there have been a few re-
ports about these catalysts’ high performance in the WGS reac-
tion at low temperature, the promoting effect of additive met-
als, the structures and surface properties of the molybdenum-
containing carbides, and the relation to catalyst performance.
In the present study, Co–Mo oxides with various compositions
were carburized at various temperatures and tested for their
The water–gas shift (WGS) reaction is being studied as a
CO-removal technology for obtaining reformed fuel from hy-
drocarbons and methanol, in view of its application to fuel
cells. Although CuO–ZnO–Al2O3 is currently used commer-
cially for the WGS reaction at low temperatures [1–6], it must
be improved and replaced by other novel catalysts that exhibit
higher resistance to sulfur compounds in the feeds and to de-
activation by oxygen and water. Other metal-based catalysts,
including Fe [7], Ni [8], Li–Mg [9], Cu/Mn [10], and Mo cat-
alysts [11–14], have been investigated to ascertain their suit-
ability for the WGS reaction. The molybdenum-containing cat-
alysts exhibited high performance at high temperature, for ex-
ample, potassium-promoted NiMo/Al2O3 [11] and Mo/Al2O3
[14] catalysts at 625–673 K. It also has been reported that the
*
Corresponding author.
E-mail address: mnagai@cc.tuat.ac.jp (M. Nagai).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.01.003

490
M. Nagai, K. Matsuda / Journal of Catalysis 238 (2006) 489–496
ability to catalyze the WGS reaction at the low temperature of
453 K. The relationship between the activities of these Co–Mo
carbides for the WGS reaction and their structures and sur-
face properties were studied by X-ray diffraction (XRD), N2
and CO adsorption, transmission electron microscopy (TEM),
temperature-programmed carburization (TPC), and X-ray pho-
toelectron spectroscopy (XPS). The 873 K-carburized Co–Mo
catalyst was also compared with the Co–Mo reduced at 873 K in
100% H2 in terms of structure and catalytic activity. The mech-
anisms of the Co–Mo carbide formation were also investigated.
2.2. The WGS reaction
The WGS reaction was carried out at 453 K in a stream
of 10.5% CO and 21% H2O in a He balance gas at a flow
rate of 60 ml/min. Water (ultra-pure) was pumped at a rate
of 0.006 ml/min using a micropump (Lab-Quatec model
LP-6300). CO, CO2, and H2 were quantitatively analyzed using
an on-line gas chromatograph with a thermal conductivity de-
tector (column, activated carbon; i.d., 6 mm; length, 2 m). The
column temperature was initially held at 413 K for 6 min, after
which it was increased from 413 to 473 K at a rate of 20 K/min,
then held at 473 K for 6 min. The reaction gas was sampled at
30-min intervals, starting 5 min after the run and ending after
300 min.
2. Experimental
2.1. Catalyst preparation
2.3. Characterization
The oxidic precursors of the Co–Mo carbides with Co/(Co+
Mo) ratios of 0, 0.10, 0.25, 0.35, 0.5, and 0.75 were pre-
pared using a mixture of an aqueous solution of cobalt nitrate
(Co(NO3)2·6H2O; Kishida Chemical Co., 99%) and an aque-
ous solution of ammonium heptamolybdate [(NH4)6Mo7O24·
4H2O; Kishida Chemical Co., 99%]. The compounds were dis-
solved in water at 353 K with stirring, producing a viscous
mixture. The solid products were placed in an oven, dried at
373 K overnight, heated at 773 K for 5 h, and then cooled to
room temperature. The oxidized precursors (0.2 g) were placed
on a porous quartz plate in a 10-nm-i.d. quartz microreactor, the
ends of which were each connected to a 1/8-inch stainless steel
tube. These were heated in dry air at a rate of 66.7 ml/min,
oxidized at 723 K for 1 h, and then cooled to 573 K. The
oxidized catalyst was purged at 573 K with He (15 ml/min)
for 10 min, carburized by the temperature-programmed reac-
tion in a stream of 20% CH4/H2 (66.7 ml/min) from 573 K
to a final temperature of 823–1023 K at the rate of 1 K/min,
and maintained at the final temperature for 2 h. The catalyst
was cooled to 453 K in flowing 20% CH4/H2 and then purged
with He for 10 min before the reaction. The carbide catalysts
were removed from the microreactor and then transferred to a
glove box, in which the atmosphere was exchanged five times
with argon (99.9999%). The carbide catalysts were then used
for characterization or the WGS reaction without exposure to
air. The 873 K-reduced Co0.5Mo0.5 catalyst was prepared by
the temperature-programmed reaction in a stream of 100% H2
(66.7 ml/min) from 573 to 873 K at a rate of 1 K/min and
maintained at 873 K in flowing H2 for 2 h. A commercial CuZn
catalyst (Süd-Chemie, AG) was also used in the reaction for
comparison. Before the reaction, the temperature of the CuZn
catalyst (0.2 g) was increased from room temperature to 473 K
at a rate of 10 K/min in a stream of 10% H2/He (15 ml/min),
maintained at 473 K for 3 h, then lowered to 453 K in flowing
He, after which the catalyst was purged with He (15 ml/min)
for 10 min. The Co–Mo carbide catalysts are identified as
CoxMo1−xC-Tj , where x and 1 − x are the percentages of Co
and Mo in the precursor compounds, respectively, and Tj is the
carburizing temperature.
The BET surface areas of the catalysts were measured us-
ing an Omnisorp 100CX (Beckman Coulter) after the catalyst
was evacuated at 473 K and 1.3 × 10−4 Pa for 2 h. The irre-
versible CO chemisorption was determined using an Omnisorp
100CX analyzer. Before the CO adsorption was measured, the
catalysts were first heated from room temperature to 673 K at
a rate of 10 K/min in a stream of He, purged at 673 K for
2 h, pretreated in a stream of H2 at 623 K for 1 h, and then
cooled to 303 K in a vacuum. For the pretreatment of CuZn,
the catalyst was heated from room temperature to 423 K in a
stream of He, purged in a stream of He for 2 h, and reduced
at 423 K for 3 h in a stream of H2. The bulk structures of
the oxide and carbide materials were measured by XRD with
a Rigaku RINT2000 using Cu-Kα radiation. The peaks were
identified on the basis of the JCPDS card references for MoO3,
MoO2, β-Mo2C, CoO, Co3O4, Co metal, and CoMoO4 (35-
609, 32-671, 35-787, 43-1004, 43-1003, 15-806, and 21-868,
respectively). Co3Mo3C was identified by comparing the fol-
lowing data: 80-0339, 2θ = 39.9, 42.4, and 46.4◦. The mapping
of the elements Co, Mo, C, and O and the surface structure
of the Co0.5Mo0.5C-873 catalyst were analyzed using a JEM-
2500SE transmission electron microscope (Japan Electronic
Co.) with an energy dispersion analyzer (EDS). TPC of Mo and
the Co0.25Mo0.75 oxides was studied to determine how the Co–
Mo oxides were carburized during the carburization. TPC was
carried out by heating from 573 to 973 K at a ramp rate of 10
K/min and maintaining the temperature at 973 K for 2 h, while
monitoring the H2O (m/z = 18) using a quadruple mass spec-
trometer (Quadstar 422, Balzer Co.). TPR was done in situ for
the Co0.5Mo0.5C-873 catalysts by heating from 873 to 1220 K
at a rate of 10 K/min in a 15-ml/min stream of H2. XPS was
performed with a Shimadzu ESCA 3200 photoelectron spec-
trometer using Mg-Kα. The experimental procedure involved
removing the catalysts from the reactor after the carburizing
treatment, transferring them to a glove box without exposure to
air, mounting them on a holder with carbon tape while in the
glove box, and then introducing them into the chamber. Argon
etching was done for 1 min before measurement at a pressure
of 5 × 10−6 Pa. The binding energies of Mo 3d and Co 2p were
analyzed at 220–240 and 775–805 eV, respectively, using the

M. Nagai, K. Matsuda / Journal of Catalysis 238 (2006) 489–496
491
Fig. 1. (A) Conversion of CO to CO over (") Co Mo C-873, ( ) MoC-
2
0.5
0.5
873, and ( ) CuZn catalyst. (B) Conversion of CO to CO at (!) 5 and (
)
2
300 min over the Co–Mo carbides with various Co/Mo ratios.
Shirley baseline correction method. The binding energies of C
1s and O 2p were measured at 282.5–292.0 (for the carbidic and
graphitic carbons) and 523–543 eV, respectively.
Fig. 2. Conversion of CO over (A) Co
ized at various temperatures at (!) 5 and ( ) 300 min.
Mo
and (B) Mo catalysts carbur-
0.25
0.75
action. CO conversion for the WGS reaction over Co0.25Mo0.75
and Mo carburized at 823–973 K at the reaction temperature
of 453 K is also shown in Fig. 2. The Co0.25Mo0.75C-873
was more active than the catalysts carburized at the other tem-
peratures. Conversion by the Co0.25Mo0.75C-873 catalyst was
higher than conversion by the MoC-923 catalyst.
These results suggest that the 25% Co content produced a
52% increase in conversion and a 50 K decrease in temperature
for carburization of the Co–Mo oxides relative to that of Mo.
Thus, the reactivity of the Co–Mo carbides catalysts with re-
spect to the WGS reaction was very sensitive to the carburizing
temperature, leading to the creation of various carbide species,
such as each or both oxycarbides and carbides of cobalt and
molybdenum. The CO chemisorption of the catalysts is charac-
terized in Table 1. CO adsorption was 2.1 times higher in the
Co0.5Mo0.5C-873 catalyst than in the CuZn catalyst. This re-
sult suggests that the Co–Mo carbide probably competed with
CuZn in catalyzing the WGS reaction. Co0.25Mo0.75C-873 and
3. Results and discussion
3.1. The WGS reaction
The conversion of CO to CO2 in the WGS reaction over
MoC-923, Co0.25Mo0.75C-873, Co0.5Mo0.5C-873, and CuZn at
453 K is shown in Fig. 1A. The conversion rate at 5 min was
higher for the Co0.5Mo0.5C-873 catalyst than for the commer-
cial CuZn catalyst, although the activity of the Co–Mo catalyst
gradually decreased, becoming less than that of the CuZn cat-
alyst at 300 min. The MoC-923 was extremely less active than
the Co0.5Mo0.5C-873 and CuZn catalysts. As shown in Fig. 1B,
the CO conversion for the Co–Mo catalysts increased with in-
creasing Co content up to 50%, and then decreased at a 75%
cobalt content. The Co0.5Mo0.5 catalyst was the most active
among the carbide catalysts with various Co contents. It is note-
worthy that the Co0.5Mo0.5C-873 catalyst was more active than
the commercial CuZn catalyst during the initial stage of the re-

492
M. Nagai, K. Matsuda / Journal of Catalysis 238 (2006) 489–496
Table 1
lyst did not have a distinct structure but was amorphous, and
Co0.75Mo0.25C-873 contained a small amount of Co3Mo3C.
The formation of this amorphous carbide led to a high surface
area and high catalytic activity with respect to the WGS reac-
tion. In addition, the patterns of the peaks for the catalysts after
the reaction were the same as those before the reaction, and no
crystals grew for 300 min during the reaction.
The BET surface areas of Mo and the Co–Mo carbides are
shown in Tables 1 and 2. The carburization of the Co–Mo ox-
ides markedly increased the surface area of Co0.1Mo0.9-873, to
76 m2/g, from the 3 m2/g of MoC-823/873. The Co0.5Mo0.5
catalyst exhibited the highest surface area (94 m2/g) of all
the bulk carbide catalysts. Korlann et al. [20] reported a BET
surface area of 111 m2/g for the 20 wt% Co3Mo3C/Al2O3
through the topotactic preparation of the Co–Mo carbide by
the temperature-programmed reaction of Co3Mo3N from 673
to 950 K in 20% CH4/H2. The incomplete carburizing prepa-
ration of the unsupported Co–Mo carbide from 573 to 873 K
for 3 h in this study can provide a high-surface area Co–Mo
carbide.
STEM images and element mappings for the total image,
molybdenum, cobalt, and carbon for the STEM analysis of the
Co0.5Mo0.5C-873 catalyst are shown in Fig. 4A. The element
image of the Mo atom overlapped that of the C atom at the dark
portion of the STEM image, indicating that almost all of the
molybdenum and carbon were equally distributed to form the
Mo carbide. For Co, the darker portion corresponded to that of
the total element mapping, showing that the presence of Co was
locally distributed on the surface of Mo2C. TEM images of the
surface structure of the Co0.5Mo0.5C-873 catalyst is shown in
Figs. 4B and C. The structure is a hexagonal closed packing of
the β-Mo2C (0001)-phase lattice, but the lattices are scattered
in parts of the Mo-rich region; see Fig. 4B (a). The amorphous
substance is seen in the spotted stripes on the β-Mo2C surface.
All of the crystals consisted mainly of β-Mo2C with amor-
phous cobalt species, such as the amorphous Co–Mo carbide.
In the Co-rich region [Fig. 4C (c)], not all lattices could be ob-
served, and the crystals were likely amorphous. On the total sur-
face, the spotted-stripe patterns could not be clearly observed,
and but they likely dominated the surface area structure of the
catalysts.
BET surface areas, CO adsorption, and TOF for 873 K-carburized catalysts
with various Co contents
b
Co
content
BET surface
area (m /g)
CO adsorption
(µmol/g)
Reaction rate
TOF
(min
2
−1
)
(mmol/(min g))
a
0
26
76
67
74
18
58
45
65
0.306
0.287
0.431
0.544
17
10
25
35
50
75
CuZn
5.0
9.6
8.4
10
9.9
25
c
c
c
94 ꢀ88 ꢁ
70 ꢀ64 ꢁ
0.719 ꢀ0.419 ꢁ
49
35
25
0.345
0.624
c
61 ꢀ60 ꢁ
a
923 K-carburized Mo.
At reaction time of 5 min.
At reaction of 300 min.
b
c
MoC-923, with high CO adsorption, exhibited high catalytic
activity (reaction rate and turnover frequency [TOF]) with re-
spect to the WGS reaction. The TOF (rate of conversion of CO
to CO2 at 5 min over the irreversible CO uptake) of the CuZn
catalyst was three times that of the Co–Mo carbides during the
reaction. The TOF of the 873 K-carburized catalyst was greater
than that of the catalysts at the other carburizing temperatures
listed in Table 2.
3.2. The bulk structure and composition of Co–Mo carbide
catalysts
The XRD patterns of the 873 K-carburized Co–Mo catalysts
with various Co/(Co + Mo) ratios are shown in Fig. 3A. All
of the carbide catalysts contained β-Mo2C. The peak intensity
of β-Mo2C decreased with increasing Co content; especially
above 35% Co content; all of the peaks became weaker, and the
Co0.5Mo0.5 catalyst consisted of small peaks of β-Mo2C and
Co metal with a very weak Co3Mo3C peak. The sharp peaks
of the Co metal with a small peak of Co3Mo3C were observed
in Co0.75Mo0.25C-873. The XRD patterns of Co0.25Mo0.75 ver-
sus carburizing temperature are shown in Fig. 3B. Carburiza-
tion of Co0.25Mo0.75 at 873–973 K produced mainly β-Mo2C,
whereas that at 823 K formed mainly MoO2. The crystallinity
of β-Mo2C was low in the active Co0.25Mo0.75C-873 catalyst,
but increased in the Co0.25Mo0.75C-973 catalyst. Newsam et
al. [18] and Liang et al. [19] reported that the carburization of
Co0.5Mo0.5 oxide with activated carbons in hydrogen at 873 K
formed a η-type Co–Mo carbide structure, identical to that in
the bimetallic carbide Co6Mo6C2 [18,19] and Co3Mo3C [20].
Co6Mo6C [18] was formed when Co(en)3MoO4 was heated
in a H2/He flow and allowed to react with CO/CO2 mixtures
at 1273 K. In this study, the active Co0.5Mo0.5C-873 cata-
The XRD pattern of Co0.5Mo0.5 reduced at 873 K in a stream
of H2 is shown in Fig. 5, and it clarifies the difference be-
tween treating the Co–Mo oxide at 873 K in a stream of H2 and
treating it in 20% CH4/H2. The reduced Co0.5Mo0.5 catalyst
exhibited the XRD peaks of Co3Mo and Mo metal at low inten-
sities without Co3Mo3C and Co metal and with the surface area
of 55 m2/g. The reduced Co–Mo catalyst exhibited no catalytic
Table 2
BET surface area, CO adsorption, and TOF for Mo and Co
Mo
carburized at various temperatures
0.25
0.75
2
−1
)
BET surface area (m /g)
CO adsorption (µmol/g)
Reaction rate (mmol/(min g))
TOF (min
Mo
Mo
Co
Mo
Mo
Co
0.25
Mo
Mo
Co
0.25
Mo
Co Mo
0.25 0.75
0.25
0.75
0.75
0.75
823
873
923
973
3
3
26
7
15
67
12
11
5
8
18
5
44
45
19
18
0.005
0.193
0.306
0.279
0.273
0.431
0.100
0.009
1
24
17
56
6.2
9.6
5.3
0.5

M. Nagai, K. Matsuda / Journal of Catalysis 238 (2006) 489–496
493
(A)
(B)
Fig. 3. XRD patterns of (A): (a) MoC-873, (b) Co
at (a) 823, (b) 873, (c) 923, and (d) 973 K.
Mo
C-873, (c) Co Mo C-873, and (d) Co
Mo
C-873, and (B): Co
Mo catalysts carburized
0.75
0.25
0.75
0.5
0.5
0.75
0.25
0.25
activity for the WGS reaction. In contrast, the Co0.5Mo0.5C-873
catalyst in the 20% CH4/H2 mixture consisted of fine particles
with a high surface area (94 m2/g). As a result, the reduced cat-
alyst did not contain the active Co–Mo carbides, which differed
in structure from the carbide catalyst.
52 K to a temperature lower than that for Mo. The carburiz-
ing temperatures at 873 K for the Co0.25Mo0.75 oxide and at
923 K for the Mo oxide were 20–22 K lower than the two high-
temperature peaks of H2O in the TPC. This result shows that
the bimetallic Co–Mo oxides were not completely carburized at
873 K and that oxygen remained in some parts, which then gen-
erated the amorphous Co–Mo oxycarbides. This result agrees
with the high WGS activity data. Combining the TPC and XRD
results, it is clear that the first desorption peak of H2O at 747 K
was due to the reduction and/or carburization of MoO3 to MoO2
and Mo oxycarbide (MoCyOz) and of CoMoO4 to Co–Mo oxy-
carbide (CoMoCyOz),
3.3. Carburization of the Co–Mo oxide
The profile of the H2O (m/z = 18) desorption during the
TPC of MoO3 and Co0.25Mo0.75, based on the suitable carbur-
ization temperature that can be determined, is shown in Fig. 6.
The desorption of H2O was observed for Mo and Co0.25Mo0.75
at two peaks of low temperature (780 and 747 K, respectively)
and high temperature (945 and 893 K, respectively), during the
TPC. The high-temperature peak for Co0.25Mo0.75 shifted by
20%CH4/H2
MoO3 −−−−−−−→ MoO2 + MoCyOz

494
M. Nagai, K. Matsuda / Journal of Catalysis 238 (2006) 489–496
(A)
Fig. 4. (A) Element of Co Mo C-873. (a) STEM imaging and mapping of (b) molybdenum, (c) cobalt, and (d) carbon. High resolution TEM photographs of
0.5
0.5
Co Mo C-873 in (B) molybdenum-rich and (C) cobalt-rich regions. Regarding (B), (b) is the color reversal of (a). Regarding (C), (d) is the color reversals of (c).
0.5
0.5
and
CH4/H2 readily formed the Co–Mo oxycarbide from CoMoO4.
Furthermore, the C/(Co+Mo) ratio for the Co–Mo carbides for
the 35 and 50% Co contents was higher than that at the 25% Co
loading, as can be seen in Table 3. The most active Co0.5Mo0.5
oxycarbide had the composition CoMoC6.2O1.4, which contains
a large amount of carbons and less oxygen than the other Co–
Mo oxycarbides, such as Co0.25Mo0.75C-873. The promotion
of WGS activity by the addition of cobalt at lower carburizing
temperatures is probably due to either the activation of hydro-
gen atoms by Co metal [19] or the Co spillover hydrogen from
H2O to the Mo atoms on the contact layers of Co. To deter-
mine the amorphous structure of Co0.5Mo0.5C-873 by XRD,
the sample was heated from 873 to 1223 K in a stream of H2
after the TPC. The peaks of Co3Mo3C were observed in Fig. 7.
It is reasonable to assume that the amorphous Co–Mo carbide
was converted to the Co3Mo3C crystallite. Consequently, the
active Co0.5Mo0.5C-873 contained only amorphous Co3Mo3C.
The formation of this amorphous Co–Mo carbide led to a high
surface area and high catalytic activity with respect to the WGS
reaction.
20%CH4/H2
CoMoO4 −−−−−−−→ CoMoCyOz.
The desorption peak of H2O in the saddle region at 747–893
K is due to the start of the carburization of MoO2 + MoCyOz
to β-Mo2C and of CoMoCyOz to Co–Mo carbide (CoMoCy),
as well as the completion of the first process of the reduction
and/or carburization. The desorption peak at 893 K was due to
the carburization of MoCyOz to β-Mo2C and subsequently the
amorphous CoMoCy with Co metal,
20%CH4/H2
MoCyOz −−−−−−−→ β-Mo2C
and
20%CH4/H2
CoMoCyOz −−−−−−−→ CoMoCy + Co metal.
Xiao et al. [21] reported the presence of a high-temperature
peak smaller than the low-temperature peak found during the
TPC preparation of Co0.5Mo0.5 oxide in a stream of the 10%
C2H6/H2 mixture. The carburizing treatment in a stream of 20%

M. Nagai, K. Matsuda / Journal of Catalysis 238 (2006) 489–496
495
(B)
(C)
Fig. 4. (continued).
Fig. 5. XRD patterns of Co Mo reduced at 873 K in a stream of hydrogen.
0.5
0.5
3.4. Active species
The Co0.5Mo0.5C-873 catalyst exhibited a high activity for
the WGS reaction and contained small CoMoCyOz particles.
This increase in activity is likely due to the increase in the
carbon-deficient sites and the Co atoms of the Co–Mo carbides.
The increase in the Co produced fine particles of the Co–Mo
oxycarbide from β-Mo2C and caused a high CO adsorption. In
a previous paper, β-Mo2C0.8 with many carbon-deficient sites
was reported to exhibit a much higher CO adsorption than the
Fig. 6. Desorption of H O during temperature-programmed carburization of (a)
2
Mo oxide and (b) Co
Mo
oxide from room temperature to 973 K at the
0.25
0.75
damp rate of 1 K/min and kept at 973 K for 2 h.
complete crystallite of β-Mo2C, according to a Monte Carlo
simulation [22]. The Co–Mo oxycarbide had many carbon-
deficient sites of Co3Mo3C crystallites. On what catalyst sites
were the CO and H2O adsorbed during the WGS reaction?

496
M. Nagai, K. Matsuda / Journal of Catalysis 238 (2006) 489–496
Table 3
but less active than CuZn at 300 min into the reaction. CO
adsorption of Co0.5Mo0.5C-873 exceeded that on the CuZn
catalyst. Based on the TPC analysis, the Co0.25Mo0.75C-873
exhibited H2O desorption peaks at 747 and 893 K, with the
former peak ascribed to the formation of Co–Mo oxycarbide
from the oxide and the latter peak ascribed to the formation
of Co–Mo carbide from the oxycarbide. The Co0.5Mo0.5C-873
contained Co–Mo oxycarbide with β-Mo2C and Co metal and
probably consisted of β-Mo2C with amorphous cobalt species.
The incomplete carburization of the Co–Mo oxide formed the
amorphous Co–Mo oxycarbide, leading to a high activity for
the WGS reaction. Water reacted with the adsorbed CO or car-
bidic carbons to the produce hydrogen and carbon dioxide in
the gas phase.
Atomic ratios of C/(Co + Mo) and O/(Co + Mo) for Co
Mo
C-873 cat-
0.25
0.75
alysts
Co content
Atomic ratio
C/(Co + Mo)
O/(Co + Mo)
0
10
25
35
50
5.3
2.5
2.1
3.0
3.1
2.4
1.7
1.4
0.9
0.7
Acknowledgments
This work was carried out as a research project using a
Grant-In-Aid for Scientific Research from the Japanese Min-
istry of Education, Sport, Culture, and Science.
References
Fig. 7. XRD patterns of Co Mo C-873 heated from 873 to 1123 K in a
0.5
0.5
stream of hydrogen after TPC.
[1] C. Rhodes, G.J. Hutchings, A.M. Ward, Catal. Today 23 (1995) 43.
[2] T. Utaka, K. Sekizawa, K. Eguchi, Appl. Catal. A 194–195 (2000) 21.
[3] Y. Tanaka, T. Utaka, R. Kikuchi, T. Takeguchi, K. Sasaki, K. Eguchi,
J. Catal. 215 (2003) 271.
[4] N.A. Koryabkina, A.A. Phatak, W.F. Ruettinger, R.J. Farrauto, F.H.
Ribeiro, J. Catal. 217 (2003) 233.
[5] S. Zhao, T. Luo, R.J. Gorte, J. Catal. 221 (2004) 413.
[6] P. Ratnasamy, D. Srinivas, C.V.V. Satyanarayana, P. Manikandan, R.S.S.
Kumaran, M. Sachin, V.N. Shetti, J. Catal. 221 (2004) 455.
[7] Y. Hu, H. Jin, J. Liu, D. Hao, Chem. Eng. J. 78 (2000) 147.
[8] Y. Li, Q. Fu, M. Flytzani-Stephanopoulos, Appl. Catal. B 27 (2000) 179.
[9] I. Balint, K. Aika, Appl. Catal. A 196 (2000) 209.
[10] Y. Tanaka, T. Utaka, R. Kikuchi, T. Takeguchi, K. Sasaki, K. Eguchi,
J. Catal. 215 (2003) 271.
[11] M. Kantschewa, F. Delannay, H. Jeziorowski, E. Delgado, S. Eder, G. Ertl,
H. Knözinger, J. Catal. 87 (1984) 482.
[12] M. Laniecki, M. Malecka-Grycz, F. Domka, Appl. Catal. A 196 (2000)
293.
[13] M.A. Segura, C.L. Aldridge, K.L. Riley, US Patent (1977) 4054644.
[14] R.N. Nickolov, R.M. Edreva-Kardjieva, V.J. Kafedjiysky, D.A. Nikolova,
N.B. Stankova, D.R. Mehandjiev, Appl. Catal. A 190 (2000) 191.
[15] P. Hou, D. Meeker, H. Wise, J. Catal. 80 (1983) 280.
[16] J. Patt, D.J. Moon, C. Phillips, L. Thompson, Catal. Lett. 65 (2000) 193.
[17] D.J. Moon, J.W. Ryu, Catal. Lett. 92 (2004) 17.
[18] J.M. Newsam, A.J. Jacobson, L.E. McCandlish, R.S. Polizzotti, J. Solid
State Chem. 75 (1988) 296.
[19] C. Liang, W. Ma, Z. Feng, C. Li, Carbon 41 (2003) 1833.
[20] S. Korlann, B. Diaz, M.E. Bussell, Chem. Mater. 14 (2002) 4049.
[21] T.-C. Xiao, A.P.E. York, H. Al-Megren, C.V. Williams, H.-T. Wang,
M.L.H. Green, J. Catal. 202 (2001) 100.
[22] M. Nagai, H. Tominaga, S. Omi, Langnuir 16 (2000) 10215.
[23] R.-L. Lo, K. Fukui, S. Otani, S.T. Oyama, Y. Iwasawa, Jpn. J. Appl.
Phys. 38 (1999) 3813.
[24] T.P.St. Clair, S.T. Oyama, D.F. Cox, Surf. Sci. 468 (2000) 62.
[25] H.H. Hwu, B.D. Polizzotti, J.G. Chen, J. Phys. Chem. B 105 (2001) 10045.
[26] H. Tominaga, M. Nagai, J. Phys. Chem. B 109 (2005) 20415.
In terms of the CO adsorption, CO preferentially adsorbed on
the carbon-deficient sites of α-Mo2C(0001) and diffused into
the domain, according to the former CO-adsorption STM re-
sults [23]. Clair et al. [24] proposed that ca. 85% of the sur-
face oxygen from split CO did not burn off as CO, but instead
diffused into the α-Mo2C(0001) bulk. This suggests that CO
molecularly adsorbs on the Co- and C-terminated surface defect
sites of the CoMo carbides, rather than dissociating on the sur-
face. As for water, Hwu et al. [25] reported that H2O molecules
dissociated into hydrogen and surface hydroxyl groups, which
decomposed into hydrogen and surface oxygen on C/W(111) at
330 K. They also suggested that the hydroxyl may well migrate
to the electron-rich carbon; the surface carbon participated on
the carbide, leading to the conversion of CO to CO2. Very re-
cently, Tominaga and Nagai [26] studied the DFT of the WGS
reaction over β-Mo2C(001) and reported that β-Mo2C disso-
ciated H2O into H and O atoms and that CO approached the
O atom to form CO2. In this study, water reacted with the ad-
sorbed CO or carbidic carbons of the catalyst to produce H2 and
CO2. It is postulated that the promoting effect of the Co atom
is the diffusion of the spillover hydrogen from H2O to the Mo
atoms on the contact layers of the cobalt.
4. Conclusion
Active Co–Mo carbides catalyzing the WGS reaction were
studied on the basis of CO, XRD, TEM, and XPS. Co0.5Mo0.5C-
873 was found to be the most active among the catalysts stud-
ied. This carbide catalyst was more active than CuZn at 5 min