Influence of water on the methanation performance of Mo-based sulfur-resistant catalysts with and without cobalt additive

Article abstract of DOI:10.1002/bkcs.10017

The activities of Mo-based and Co-containing Mo-based catalysts for sulfur-resistant methanation in the presence and the absence of water are compared. When water was added to the Mo-based catalyst, its methanation activity decreased and the activity could not be recovered even after the water was removed from the system. However, for the Co-containing Mo-based catalysts, the formation of Co₉S₈ improved not only the methanation activity of the catalyst as active sites but also the stability of the catalyst especially in water-containing hydrogenation. The deactivation of the Mo-based catalyst in the presence of water is mainly due to reduction of the external acid sites and fewer molybdenum sulfide (MoS₂) stacks. The addition of Co protects the active MoS2 phase, thereby preventing the deactivation of the catalyst in the presence of water.

Full text of DOI:10.1002/bkcs.10017

Article
DOI: 10.1002/bkcs.10017
BULLETIN OF THE
H. Wang et al.
KOREAN CHEMICAL SOCIETY
Influence of Water on the Methanation Performance of Mo-Based
Sulfur-Resistant Catalysts with and without Cobalt Additive
*
*
Haiyang Wang, Can Lin, Zhenhua Li, Baowei Wang, and Xinbin Ma
Key Lab for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and
Technology, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University,
Tianjin 300072, China. *E-mail: zhenhua@tju.edu.cn; xbma@tju.edu.cn
Received June 9, 2014, Accepted September 15, 2014, Published online January 5, 2015
The activities of Mo-based and Co-containing Mo-based catalysts for sulfur-resistant methanation in the pres-
ence and the absence of water are compared. When water was added to the Mo-based catalyst, its methanation
activity decreased and the activity could not be recovered even after the water was removed from the system.
However, for the Co-containing Mo-based catalysts, the formation of Co9S8 improved not only the methana-
tion activity of the catalyst as active sites but also the stability of the catalyst especially in water-containing
hydrogenation. The deactivation of the Mo-based catalyst in the presence of water is mainly due to reduction
of the external acid sites and fewer molybdenum sulfide (MoS2) stacks. The addition of Co protects the active
MoS2 phase, thereby preventing the deactivation of the catalyst in the presence of water.
Keywords: Sulfur-resistant, Methanation, Deactivation, Water
Introduction
addition, water may cause sulfate species to form; these then
cover the active phase and reduce the catalytic activity.9
Cobalt molydates are important components of catalysts
used for the partial oxidation of hydrocarbons10 and for the
synthesis of HDS catalysts.11 The desulfurization and denitro-
genation performance of MoS2 catalysts can be substantially
improvedbytheadditionofacobaltpromoter.Itisbelievedthat
the Co atoms are mostly localized on the S-edges, although in
the case of a high Co/Mo ratio, some Co atoms are also located
on the Mo-edges.12 Co atoms can substitute for Mo atoms at
theseedges, which results inS-vacancies sincethe Coions have
a lower valency than the Mo ions.13 This generates new active
sites.Itisgenerallyacceptedthatthisso-calledCo–Mo–Sphase
(structure),inwhichtheCoatomsarelocatedontheedgesofthe
MoS2 particles, is the active phase.14
Feed stock gases from a coal gasifier contain a certain
amount of water. And the sulfur-resistant methanation
(2CO + 2H2 ! CH4 + CO2) that occurs on Mo-based catalyst
is actually a sum of the methanation (3H2 + CO ! CH4 +
H2O) and water gas shift reaction (CO + H2O $ CO2 + H2).
This means that the catalysts should be active in a water-
containing environment. Therefore, it is necessary to investi-
gate the effect of water on the methanation performance of
Mo-based sulfide catalysts and to elucidate the change of
active sites on the catalysts in the presence of sulfur. In addi-
tion, the effect of adding Co to the Mo-based catalyst on the
stability of the catalyst in the presence of water vapor is also
investigated in this work. The catalysts are characterized for
their surface area, crystallinity, cluster dimension, and surface
chemicalcomposition. Thisstudyprovidesinformationforthe
design of catalysts with better activities and stabilities for sul-
fur-resistant methanation of gas feedstocks containing water.
Today, the global need for vast amounts of energy is a press-
ing concern. In addition, methods to utilize coal in an environ-
mentally friendly manner are also greatly desired. With both
goals in mind, methanation is considered an important means
of producing substitute natural gas from biomass or coal.1
Often the catalysts for the methanation of syngas contain nickel
or cobalt as the active ingredient. These metallic catalysts are
relatively cheap and very active for the reaction. However, they
are extremely sensitive to poisoning by sulfur compounds.2
Thus attention has turned to Mo-based methanation catalysts
which are sulfur-resistant and can tolerate the gases of low
H2/CO ratios that come from gasifiers. It is possible for a
Mo-based catalyst to cover the shortage of their relatively
low activity by using simplified production technology without
enhancing the H2/CO mole ratio or removing sulfur species.
Mo-based sulfides are used widely in hydrotreating pro-
cesses toproducecleanfuelsand aromatic compounds.3 These
catalysts are also used extensively in commercial catalytic
hydrogenations4,5 (e.g., hydrodesulfurization (HDS), hydro-
denitrogenation, and hydrotreating). It has been well estab-
lished that their active sites are located on the edges of the
molybdenum sulfide (MoS2) nanocrystallites, which corre-
spond to the (100) edge planes of their layered structures.6
Water can deactivate sulfided hydrotreating catalysts by
changing their structure or chemical composition. The pres-
ence of water can modify the structure of the active edges of
the sulfide phase and can influence the stability of the cata-
lysts.7 During the hydrotreating of low-sulfur content feeds,
sulfided catalysts usually undergo a continuous deactivation
as a result of the partial reoxidation of the sulfide phase.8 In
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Influence of Water and Cobalt on Methanation
Experimental
KOREAN CHEMICAL SOCIETY
samples were outgassed in the pretreatment chamber for 1 h
under ultrahigh vacuum and then transferred into the analysis
chamber. The XPS analysis for the catalyst was performed
using a PHI-1600 ESCA spectrometer equipped with mono-
chromic Mg Kα X-ray radiation. The pressure of the analysis
chamber was maintained at 2 × 10−10 Torr. The binding ener-
gies (BEs) were calibrated using the C 1s line at 284.6 eV. The
errorinthe BEswiththisreferencewasestimatedtobe 0.1 eV.
The peak area intensities were measured by planimetry of the
graphic displays of the spectra assuming Shirley baselines.
Catalytic Activity Evaluation. Catalytic performance eva-
luations were carried out in a continuous-flow, fixed-bed reac-
tor. The stainless steel reactor (70 cm in length and 12 mm in
internal diameter) was heated by an enclosed electric furnace.
The temperature was controlled by using three K-type thermo-
couples placed in the furnace, and the reaction temperature
was monitored using a K-type thermocouple placed in the
middle of the 3 mL catalyst bed (0.43–0.85 mm particle size).
The catalysts were sulfurized using a 3 vol% H2S/H2 gas mix-
ture at 400 ꢀC for 4 h before activity testing.
The catalytic reaction was conducted under the following
conditions: A mixed gas (CO/H2/N2 = 2:2:1 in volume ratio)
with1.2vol%H2Swasusedasthefeedgas. Thespacevelocity
was 5000 h–1, the reaction temperature was 560 ꢀC, and the
reaction pressure was 3 MPa. Water was injected into the
vaporizer by a micro piston pump and mixed with the feed
gas. Then a mixture containing 10 vol% water vapor was sup-
plied to the reactor to investigate the effect of water.
The outlet gases were analyzed online using an Agilent
7890A gas chromatograph equipped with six columns (three
Porapak-Q and two Carboxen (Restek, Bellefonte, Pennsylva-
nia, USA) and one capillary column (Agilent Technologies
Inc., Santa Clara, California, USA)) and three detectors
(two thermal conductivity detectors and one flame ionization
detector), using N2 or H2 as the carrier gas. The compositions
of CO, CO2, CH4, H2, and N2 were measured and then used to
calculate the H2 conversion, CO conversion, CH4 selectivity,
and CO2 selectivity according to the following formulas:
Catalyst Preparation. A commercially available γ-Al2O3
support (Yixing, China) was used to prepare the catalysts.
Catalysts with 25 wt% MoO3/Al2O3 (283 m2/g) and 5 wt%
CoO–25 wt% MoO3/Al2O3 were prepared via a conventional
incipient wetness impregnation method by impregnating the
support with an aqueous solution of ammonium heptamolyb-
date or an aqueous solution of ammonium heptamolybdate
and cobalt nitrate, respectively. The impregnated samples
were dried at 30 ꢀC for 24 h in air, and then at 120 ꢀC for 6 h,
and finally calcined at 600 ꢀC for 4 h with a heating rate of
3 ꢀC/min.ThesecatalystsaredenotedasMo/AlandCo–Mo/Al.
Catalyst Characterization. Nitrogen physisorption analysis
of the prepared catalysts was performed at −196 ꢀC on a Tris-
tar-3000 apparatus (Micromeritics, Atlanta, Georgia, USA) to
obtain the textural properties of the catalysts (specific surface
area and pore volume). Prior to measurement, the sample was
degassed at 300 ꢀC for 3 h under vacuum. The surface areas
were determined by physical adsorption of N2 at liquid nitro-
gen temperature, using the Brunauer–Emmett–Teller (BET)
equation. The pore volumes and pore size distributions were
obtained from the desorption curves of the isotherms using
the Barrett–Joyner–Halanda (BJH) model.
Thetemperature-programmed reduction (TPR) andtemper-
ature-programmed desorption (TPD) profiles of the catalysts
were obtained using an AutoChem 2910 analyzer
(Micromeritics)equippedwithathermal conductivitydetector
(TCD) for measuring the hydrogen consumption. In the TPR
experiments, a stream of gas with the composition H2/Ar = 1:9
at a flow rate of 30 mL/min was introduced into the catalyst
sample (0.20 g). The catalyst was then heated from 60 to
1000 ꢀC at a heating rate of 10 ꢀC/min.
For each of the TPD runs, 0.20 g of sample was first treated
at 200 ꢀC for 1 h and then cooled to 50 ꢀC under an argon flow.
Ammonia (1 mL) was injected into the Ar at 50 ꢀC, and the
injection was repeated for 3–5 times in order to fill the adsorp-
tion sites. TPD was then performed by heating the samples at
the rate of 10 ꢀC/min under 30 mL/min Ar flow.
X-ray diffraction (XRD) measurements were performed on
a D/max-2500 X-ray diffractometer (Rigaku, Japan) with a
Cu Kα radiation source (λ = 1.54056 Å). The scan speed
was 8ꢀ/min, with a scanning angle range of 5–85ꢀ. The phase
identifications were determined by comparison with the Joint
Committee on Powder Diffraction Standards (JCPDS).
Before transmission electron microscopy (TEM) analysis,
the catalysts were kept under dry vacuum conditions. The
TEM specimens were prepared by ultrasonically dispersing
the catalysts in ethanol, and then placing drops of the suspen-
sions onto a micro-mesh copper grid. The morphology and
structure of the catalysts were characterized by a Tecnai G2
F20 (200 kV) transmission electron microscope (FEI, Eindho-
ven, Noord-Brabant, Netherlands), with a high resolution of
0.15 nm/200 kV.
nðH2inÞ-nðH2out
Þ
XH2 =
× 100%
× 100%
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
nðH2in
Þ
nðCOinÞ-nðCOout
Þ
Xco
=
=
=
nðCOinÞ
nðCH4outÞ-nðCH4in
Þ
SCH
× 100%
4
2
nðCOinÞ-nðCOout
Þ
nðCO2outÞ-nðCO2in
Þ
SCO
× 100%
nðCOinÞ-nðCOout
Þ
nðC2H6outÞ−nðC2H6in
Þ
× 100%
SC = 2∗
2
H6
nðCOinÞ−nðCOout
Þ
whereXH andXCO refertotheH2 andCOconversions, respec-
2
tively, and SCH , SCO , and SC H refer to the CH4, CO2, and
C2H6 selectivities, respectively.
Prior to X-ray photoelectron spectroscopy (XPS) analysis,
thesamples werepretreated underdry vacuumconditions. The
4
2
6
2
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Influence of Water and Cobalt on Methanation
Results and Discussion
KOREAN CHEMICAL SOCIETY
conversion of H2 was ~10% higher than the conversion of CO.
The CO and H2 conversions decreased with time for both cat-
alysts, but the decline on the Co–Mo/Al catalyst was less. In
addition, the CO and H2 conversions on the Co–Mo/Al cata-
lyst were higher than those on the Mo/Al catalyst, indicating
that the Co has a promoting effect on the Mo-based catalyst.
The CO2 and CH4 selectivities on the Mo-based catalysts were
both ~50%.
Catalyst Activity Evaluation. The sulfided catalysts were
exposed to two different conditions for studying the effect
of water: one is methantion without water added (Blank test),
and the other is water addition to the feed gas during reaction
(with water). The methanation activities in the presence and
the absence of water were tested for about 40 h, and the results
are shown in Figure 1 (Mo/Al) and Figure 2 (Co–Mo/Al). For
both catalysts, the effect of water was tested by adding 10%
water to the reaction after 5 h of methanation with no water
present. The water was continually added from 5 to 25 h,
and then it was removed after 25 h to test the recovery of
the methanation activity.
Comparing Figures 1 and 2, a clear differentiation between
two catalysts canbefound. TheMo/Alcatalyst showed around
50% CO conversion, but CoMo/Al showed 60% CO conver-
sion with almost same selectivities.
In the absence of water, the activity of the Co–Mo/Al cata-
lyst had the same regularity as that of the Mo/Al catalyst. The
When water was added, for both catalysts the CO conver-
sion did not decline much, but the H2 conversion decreased
greatly. This is due to the water gas shift reaction, as indicated
by the increase in the CO2 selectivity. After the water was
stopped, most of the methanation activity of the Co–Mo/Al
catalyst was recovered but the methanation activity of the
Mo/Al catalyst was not. Previously, it had been reported that
the CO uptake by the sulfide sites on both the Mo and Co–Mo
catalysts strongly decreased after water addition.13,15 As
shown in Figure 3, the Co–Mo/Al catalyst showed less deac-
tivation (about 2%) than the Mo/Al catalyst (about 6%). This
implies that the addition of Co inhibits the deactivation caused
80
60
40
80
60
40
20
test with 10% H2O(g)
20
test with 10% H2O(g)
X(CO)
X(CO)
X(H2)
X(H2)
0
0
0
5
10
15
20
25
30
35 40
0
5
10
15
20
25
30
35
40
Time /h
Time /h
60
50
40
30
20
10
0
60
50
40
30
20
10
0
test with 10% H2O(g)
S(CO2)
S(CH4)
S(C2H6)
test with 10% H2O(g)
S(CO2)
S(CH4)
S(C2H6)
0
5
10
15
20
25
30
35
40
0
5
10
15
20
25
30
35
40
Time /h
Time /h
Figure 2. Effect of water on the methanation activity and selectivity
of the Co–Mo/Al catalyst: with water (solid symbols) and blank test
(empty symbols).
Figure 1. Effect of water on the methanation activity and selectivity
of the Mo/Al catalyst: withwater (solid smbols) and blank test (empty
symbols).
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Influence of Water and Cobalt on Methanation
KOREAN CHEMICAL SOCIETY
by water, and the Co atoms may help in protecting the MoS2
active sites.
The Mo/Al catalyst used in the presence of water had a
smaller pore volume and average pore diameter than the cat-
alyst used with no water. This could be related to the deac-
tivation effect of the water vapor. This effect is probably due
to the formation of a sulfate layer that covers the active
phase and/or to structural changes in the alumina support.
In contrast, the Co–Mo/Al catalyst used with water showed
no distinctive textural changes. This suggests that the Co
additive effectively modified the interactions between the
active phase and the support in such a way that the original
textural properties of catalyst were maintained when water
was added to the system. So the methanation activity of
the Co–Mo/Al catalyst changed little between reacting with
and without water.
The TPR patterns of fresh Mo/Al and the Co–Mo/Al cata-
lysts are presented in Figure 4. Both samples have two reduc-
tion peaks. The first peak corresponds to the partial reduction
of Mo6+ to Mo4+, and the second peak to the reduction of Mo4+
toMo0.16 ThereductionpeaksfortheCo–Mo/Alcatalystareat
slightly lower temperatures than those for Mo/Al, which indi-
cates that the species are easier to reduce. This is due to the
polarization effects of Co2+ on the Mo6+ terminal oxygen
atoms.17 The degree of reduction was evaluated bythe integra-
tion of the TPR curves between 60 and 600 ꢀC (the reaction
temperature was 560 ꢀC) and found to be 65.1 for the Co–
Mo/Al catalyst and 60.3 for the Mo/Al catalyst. This result
is in good agreement with the methanation activities, and it
presents a promotion of Co on the active sites.
Characterization of Mo-Based Catalysts. The textural
properties of the Mo/Al and Co–Mo/Al catalysts were deter-
mined using N2 adsorption–desorption isotherms, and the
results are listed in Table 1. Compared with the fresh catalyst,
the surface areas of both catalysts decreased greatly after sul-
fidation. However, the surfaces areas changed little after the
activity tests with and without water. The difference in the tex-
tural properties between the two catalysts can be attributed to
theaddition ofCo. TheCopromoter may blockthemicropores
of the catalysts and make the average pore diameter larger and
the surface area smaller. When the surface area and pore vol-
ume of the catalyst supports are compared to the activity
results, it appears that they are not critical factors that affect
the methanation activity.
80
Co-Mo/Al
60
Mo/Al
40
The XRD spectra of thedifferent Mo/Al and Co–Mo/Al cat-
alysts are shown in Figure 5. The sulfided catalysts were
obtained by sulfurizing the oxide catalysts with 3% H2S/H2
(100 mL/min) at 560 ꢀC for 4 h. The XRD spectra of the
Mo/Al catalysts in Figure 5 shows several broad diffraction
peaks corresponding to γ-Al2O3 (2θ = 33.4ꢀ, 37.5ꢀ, 45.7ꢀ,
59.1ꢀ, and 66.8ꢀ).18 No obvious XRD peaks specific to Mo
sulfides were detected, suggesting that the Mo species may
be present as small nano-sized particles that are well dispersed
on the support γ-Al2O3.19 Figure 5 confirms the presence of
20
0
with water
blank test
Figure 3. Influence of H2O on the catalyst deactivation.
Table 1. Textural properties of the catalysts.
BET
Average
surface Pore
pore
area
volume
diameter
(nm)
Catalyst
Mo/Al
(m2/g)
(cm3/g)
Mo/Al
Fresha
223
165
154
153
204
154
133
141
0.33
0.25
0.26
0.14
0.27
0.21
0.2
5.8
6
Sulfidedb
Spent blank testc
Spend with H2Od
6
490 oC
485 oC
957 oC
Co-Mo/Al
4.2
5
Co–Mo/Al Fresha
Sulfidedb
5.1
6
Spent blank testc
Spent with H2Od
892 oC
0.22
5.8
a As-prepared catalyst.
200
400
600
800
1000
b After sulfided at 560 ꢀC.
c After the blank test.
Temperature /oC
d After the test with water.
Figure 4. TPR profiles of the fresh catalysts.
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MoS2 peak(2θ = 14.1ꢀ)appearing inthesulfurized Co–Mo/Al
catalysts, so the crystalline MoS2 phases were detected by
XRD. There were no changes in the structure of the aluminum
support when cobalt was added.
When water vapor was added to the reaction feed, H2 was
produced inside the reactor, which transformed the crystalline
MoS2 phases to well-dispersed amorphous MoS2 nanocrystal-
lites.20 Consequently, the MoS2 peak disappeared after the
methanation test with water.
excellent catalytic performance and good stability as active
sites for the reaction, as shown in Figure 2.
Previously, TEM had been used to show that sulfided Mo/
Al and Co–Mo/Al catalysts contain black, thread-like fringes
that correspond to the MoS2 slabs.24 To compare the distribu-
tion of the slabs lengths and stacking after the sulfidation treat-
ment, and the methanation reactions with and without water,
about 15 images and 200 slabs for each sample were statisti-
cally analyzed. The distributions of the slab lengths in the Mo/
Al and Co–Mo/Al catalysts are presented in Figure 6. The
average size and the number of slabs in a stack (n) of MoS2
particles are summarized in Table 2. Representative TEM
images of Mo/Al and Co–Mo/Al catalysts are presented in
Figures 7 and 8.
Diffraction peaks for Co9S8 (2θ = 29.9ꢀ and 52.0ꢀ) are
observed in all the sulfided Co–Mo/Al samples. And the
Co9S8 crystals are decorated with MoS2 stacks, which makes
them longer.21 This, combined with the catalytic activity
results, suggests that the Co9S8 structures are beneficial for
the methanation activity. Moreover, it had previously been
reported that Co9S8 can enhance the stability of catalysts in
hydrotreating reactions.22,23 The disappearance of the sulfide
species such as MoS2 and Co9S8 could be attributed to oxida-
tion when water is present during the reaction. The well-
dispersed Co–Mo sulfides, including Co9S8, exhibited
Asshown inTable2, the MoS2 slabs became longer forboth
the Mo-based catalysts after 40 h of methanation reaction.
This is because some of the amorphous MoS2 particles grew
intowell-crystallizedstacksathigh-temperaturecalcination.25
40
Mo/Al
sulfided
30
20
10
0
blank test
with water
Al2O3
1
2
3
4
5
6
7
8
Slab length /nm
a
b
c
40
30
20
10
0
Co-Mo/Al
sulfided
blank test
with water
10
20
30
40
2θ / o
50
60
70
80
MoS2
Co9S8
Co-Mo/Al
1
2
3
4
5
6
7
8
Slab length /nm
CoMoO4
Al2O3
Figure 6. Effect of water on the slab lengths of the catalyst obtained
from TEM analysis.
a
Table 2. Theaverageslablength(L)andnumberofslabs (n)inastack
of the MoS2 particles for the Mo/Al and Co–Mo/Al catalysts
determined from TEM micrographs.
b
c
Mo/Al
L (nm)
Co–Mo/Al
Catalyst
n
2.2
1.6
1
L (nm)
n
2.5
2
Sulfided at 560 ꢀC
After the blank test
After the test with water
3.1
4.1
3.2
4.5
4.5
4
10
20
30
40
2θ / o
50
60
70
80
Figure 5. XRD patterns of the catalysts: (a) After sulfided at 560 ꢀC,
1.8
(b) after the blank test, and (c) after the test with water.
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Figure 7. Representative TEM images of different Mo/Al catalysts: (a) After sulfided at 560 ꢀC, (b) after the blank test, and (c) after the test
with water.
Figure 8. Representative TEM images of different Co–Mo/Al catalysts: (a) After sulfided at 560 ꢀC, (b) after the blank test, and (c) after the test
with water.
When water was added during the reactions, the slabs became
shorter under the severe reaction conditions. So, longer slabs
and more stacking are not responsible for the higher activity.
When water is added during the methanation reaction, the
MoS2 surface is alternately oxidized by water and then
reduced by CO.26 For the Mo/Al catalyst, after the methana-
tion reaction with water, the number of stacks in the MoS2
slabs decreased from 2.2 to 1.0 and the slab length became
smaller. This decrease may be due to the exchange of oxygen
for sulfur at the outer layer of the sulfide slabs in the presence
of water.27 The formation of an oxy-sulfide outer layer led to
a loss of the crystallinity in the external sulfide slab layer.
This is accompanied by a decrease in the MoS2 particle size,
which is responsible for the absence of diffraction fringes at
the outer layer. The decrease in the number of sulfide edge
sites resulted in the decrease in the CO uptake on the catalyst,
as well as the decrease in the methanation activity when water
was added.
As shown in Figure 8, clear MoS2 crystallites can be seen on
the sulfided Co–Mo/Alsample, which is in agreement with the
XRD results. In this catalyst, MoS2 acts as a frame and the Co
atoms decorate the MoS2 edge sites to form the active phase,
which is a highly dispersed cobalt sulfide (Co9S8).28 After the
methanation reactions in both the absence and the presence of
water, the average lengths of the MoS2 slabs and the average
number of layers in the stacks are fairly close to those in
the sulfided sample (Table 2). These results show that Co
stabilized the sulfide phase and thus the methanation activity
also improved.
The presence of strong Lewis (coordinatively unsaturated
metal atoms) and Brönsted (proton donor groups) acid sites
on a catalyst surface is important for their adsorptive and cat-
alytic applications. The TPD profiles of the catalyst that
reacted in the absence and the presence of water are presented
in Figure 9. The profiles for the Mo/Al catalyst show that, in
the presence of water, the intensity of the profile decreased
greatly, which means that the number of acid sites decreased
aswell. Thiscorresponds withthedecreasedactivity ofthecat-
alyst in the presence of water. For the Co–Mo/Al catalyst, the
decreaseinthenumberacidsitesinthepresenceofwaterisless
significant.
During the impregnation and calcination processes, molyb-
denum is anchored to the Brönsted acid sites (aluminum) and
Mo–O–Al groups are formed.29,30 Water can absorb on the
unsaturated surface molybdenum sites, which can lead to
S–O exchanges. This strongly changes the surface. From
the changes in MoS2 stacking (TEM results), it is proposed
that some of the molybdenum species are bonded to the cata-
lyst surface by alumina hydroxyl groups, forming Mo–O–
Al bonds.
XPS analysis wasused tofurther study the effect of water on
the surface compositions of the two catalysts (Mo/Al and Co–
Mo/Al). The ratio of AMo3d/AAl2p is an indication of the disper-
sion of Mo. The Mo/Al ratio was 0.11 for Mo/Al and 0.18 for
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163.9 0.1 eV indicates the presence of a MoOxSy phase
(S22−). All samples contain a large amount (82–87%) of S2–.
A sulfate species at 169.2 0.2 eV is also observed, and this
could be attributed to the reoxidation of sulfide ions when
the sample is exposed to air or water during storage and
transfer.33
Mo/Al
blank test
The relative intensities of the XPS peaks were used to esti-
mate the fractional distribution of different Mo species on the
catalyst surfaces.34,35 Table 3 lists the percentages of the dif-
ferent atom states present in the catalysts. The Mo4+ species
accounted for about 80% of the Mo species in the Co–Mo/
Al catalyst, whereas for the Mo/Al catalyst it was below
75%. The Mo/Al catalyst had a lower percentage of the sulfide
phase (Mo4+), which is in agreement with other observations.
The contents of both Mo4+ and Mo5+ decreased in the sample
that reacted in the presence of water. So the formation of the
inactive species occurred in both disulfide and oxysulfide
intermediates, while these species were not promoted with
Co atoms. And molybdenum phases with lower valence states
were slightly oxidized.
with water
400
100
200
300
500
600
Temperature / °C
Co-Mo/Al
blank test
For the Co–Mo/Al catalyst, the Mo distribution on the
used catalysts was not greatly affected by the presence of
water, indicating that no significant changes in the Mo
valence states occurred during the methanation process with
water. The phase changes of the Mo species were not so
severe, which means the Co–Mo–S phase and Co9S8 species
may enhance the activity of Mo-based catalyst for the hydro-
genation reaction as active sites in the presence of sulfur.
Furthermore, the stable distribution of Mo shows that
the formation of inactive species was inhibited by the
Co-promoted catalyst.
with water
100
200
300
400
500
600
Temperature / °C
Figure 9. TPD profiles of the catalysts reacted in the absence and the
presence of water.
Conclusion
Adding water to a Mo–Al catalyst decreased the methanation
activity, and the activity could not be recovered after the
water was removed from the system. In contrast, for a Co–
Mo/Al catalyst, the methanation activity that was lost upon
the addition of water was mostly recovered. The Co improved
not only the methanation activity of the catalyst but also the
catalytic stability of the catalyst especially in a water-
containing atmosphere. The Co additive inhibited the sul-
fur–oxygen exchanges that occur in the presence of water.
In addition, cobalt sulfide species (Co9S8) and Co-decorated
MoS2 stacks protect the active MoS2 stacks. The formation of
these structures protects the surface acid sites and the MoS2
stacks. These results give a better understanding of the metha-
nation performance of sulfur-resistant catalysts in water-
containing environments.
Co–Mo/Al, and these ratios did not change after water was
added. This indicated that no loss of molybdenum occurred.
The spectra shown in Figures 10 and 11 show the Mo and
S states that are present on the surfaces of the used catalysts.
These are useful to determine the extent of sulfidation and
the distribution of the active species on the support. The
molybdenum exists mainly as disulfide (MoS2, Mo4+), as indi-
cated by the peaks at 229.0 0.1 and 232.2 0.1 eV. The
peaks at232.7 0.1and 235.8 0.2 eVaredue tothepresence
of a Mo oxide phase (MoOx, Mo6+). The other phase is an
intermediate Mo oxysulfide (MoOxSy, Mo5+),31 as indicated
by the doublet at 230.2 0.1 and 233.4 0.1 eV. The BEs
of Mo 3d and S 2s partly overlapped each other. In this case,
we attributed the overall contribution of S 2s to the peak
located at 226.1 0.1 eV (MoS2) and 227.2 0.2 eV
(MoOxSy).
TheresultsinFigure11indicate thatthesulfurexistedasS2–
in MoS2, CoMoS, or Co9S8 as indicated by the peaks at 161.8
0.1 and 162.7 0.1 eV. These are close to the values
reported in the literature.32 The doublet at 163.2 0.1 and
Acknowledgment. The authors gratefully acknowledge the
financial support from Tianjin Municipal Science and Tech-
nology Commission (14JCZDJC37500).
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KOREAN CHEMICAL SOCIETY
(a)
(b)
Mo(IV)3d5/2
Mo(IV)3d5/2
Mo(IV)3d3/2
Mo(VI)3d5/2
Mo(IV)3d3/2
Mo(VI)3d3/2
S22-
S2-
Mo(VI)3d5/2
Mo(VI)3d3/2
Mo(V)3d5/2
S2-
S22-
Mo(V)3d5/2
Mo(V)3d3/2
Mo(V)3d3/2
240 238 236 234 232 230 228 226 224 222
Binding energy /eV
240 238 236 234 232 230 228 226 224 222
Binding energy /eV
Mo(IV)3d5/2
(c)
(d)
Mo(IV)3d5/2
Mo(IV)3d3/2
Mo(IV)3d3/2
Mo(VI)3d5/2
Mo(VI)3d3/2
Mo(VI)3d5/2
Mo(V)3d5/2
S22-
S2-
Mo(VI)3d3/2
S2-
S22-
Mo(V)3d3/2
Mo(V)3d3/2
Mo(V)3d3/2
240 238 236 234 232 230 228 226 224 222
Binding energy /eV
240 238 236 234 232 230 228 226 224 222
Binding energy /eV
Figure 10. High-resolution XPS Mo 3d spectra: (a) Mo/Al catalyst after the blank test, (b) Mo/Al catalyst after the test with water, (c) Co–Mo/Al
catalyst after the blank test, (d) Co–Mo/Al catalyst after the test with water.
S2-
(b)
(a)
S2-
S2-
S2-
2p3/2
2p3/2
S2-
S22-
2-
2p1/2
S
2 2p3/2
S2-
S
2-
2p1/2
2p1/2
S2
222- p
S22-
S22-
2p1/2
3/2
166
165
164
163
162
161
160
159
166
165
164
163
162
161
160
159
Binding energy /eV
Binding energy /eV
(d)
(c)
S2-
S2-
S2-
S
2- 2p3/2
2p3/2
S2-
2p1/2
S2-
2p1/2
S22-
2p3/2
222- p
2-
2-
S2
S
S2
S22-
2p3/2
2-
1/2
S
2 2p1/2
166
165
164
163
162
161
160
159
166
165
164
163
162
161
160
159
Binding energy /eV
Binding energy /eV
Figure 11. High-resolution S 2p spectra by XPS: (a) Mo/Al catalyst after the blank test, (b) Mo/Al catalyst after the test with water, (c) Co–Mo/Al
catalyst after the blank test, and (d) Co–Mo/Al catalyst after the test with water.
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Table 3. Surface structure of the Mo-based catalysts after reaction with or without water.
Mo4+ Mo5+ Mo6+
BE (eV) BE (eV) BE (eV)
S2–
S22−
Catalyst
%atom
74.36
65.70
77.15
80.62
%atom
6.50
%atom
19.15
23.96
16.35
16.08
BE (eV)
161.8
161.8
161.9
161.8
%atom
80.87
85.02
33.33
33.33
BE (eV)
163.1
163.2
163.3
163.2
%atom
19.13
14.98
66.67
66.67
Mo/Al
229.0
229.0
229.0
229.1
230.2
230.2
230.2
230.2
232.6
232.8
232.6
232.6
Mo/Ala
10.34
6.50
Co–Mo/Al
Co–Mo/Ala
3.30
a Methanation reaction with water added.
18. X. Wang, G. Liu, J. Yu, A. E. Rodrigues, Appl. Catal. A Gen.
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