The effect of citric acid on the catalytic activity of nano-sized MoS2 toward sulfur-resistant CO methanation

Article abstract of DOI:10.1002/aoc.4339

In this work, a facile hydrothermal route was used to prepare nano-sized MoS₂ catalyst. The effect of citric acid during the MoS₂ preparation process on the catalytic activity of sulfur-resistant CO methanation was investigated. It was found that citric acid played an adverse role on the catalytic activity of MoS₂ toward sulfur-resistant CO methanation. However, CO methanation performance turned out to be better when NH₂OH?HCl as a reductant was removed during the catalyst preparation process. The X-ray diffraction (XRD) and infrared spectroscopy (IR) were performed to discuss the possible mechanism for the effect of citric acid towards CO methanation performance.

Full text of DOI:10.1002/aoc.4339

Received: 8 December 2017
DOI: 10.1002/aoc.4339
Revised: 2 February 2018
Accepted: 3 February 2018
C O M M U N I C A T I O N
The effect of citric acid on the catalytic activity of nano‐sized
MoS toward sulfur‐resistant CO methanation
2
Zhenhua Li
| Zijia Yin
| Weihan Wang | Ye Tian | Baowei Wang | Xinbin Ma
Key Laboratory for Green Chemical
Technology of Ministry of Education,
School of Chemical Engineering and
Technology, Collaborative Innovation
Center of Chemical Science and
Engineering (Tianjin), Tianjin University,
Tianjin 300072, China
In this work, a facile hydrothermal route was used to prepare nano‐sized MoS2
catalyst. The effect of citric acid during the MoS preparation process on the
2
catalytic activity of sulfur‐resistant CO methanation was investigated. It was
found that citric acid played an adverse role on the catalytic activity of MoS
2
toward sulfur‐resistant CO methanation. However, CO methanation perfor-
mance turned out to be better when NH OH⋅HCl as a reductant was removed
2
Correspondence
Li Zhenhua, School of Chemical
Engineering and Technology, Tianjin
University, 13820514923, China.
Email: lizhenhua001@gmail.com
during the catalyst preparation process. The X‐ray diffraction (XRD) and
infrared spectroscopy (IR) were performed to discuss the possible mechanism
for the effect of citric acid towards CO methanation performance.
Xinbin Ma, School of Chemical
Engineering and Technology, Tianjin
University Tianjin, China
KEYWORDS
2
citric acid, hydrothermal synthesis, molybdenum‐citrate complex, MoS , sulfur‐resistant methanation
Email: xbma@tju.edu.cn
Funding information
National High Technology Research
and Development Program of China (863
Project), Grant/Award Number:
2015AA050504; National Natural Science
Foundation of China, Grant/Award Num-
ber: 21576203, 21606167
1
| INTRODUCTION
simple manipulation. In the hydrothermal process, addi-
tives were normally introduced in order to obtain MoS2
particles with better properties. For example, it has been
Over the past few years, inorganic nanomaterials have
drawn much attention because of their functional proper-
ties and potential applications, such as electrochemical
hydrogen storage, supercapacitor, magnesium batte-
ries and industrial catalysts. Nano‐sized transition
reported that MoS nanomaterials could be obtained by
2
[
11]
adding silicontungstic acid as an additive.
Meanwhile,
[1]
[2]
it was also reported that cetyl trimethyl ammonium bro-
[3]
[4]
[12]
mide (CTAB) could assist hydrothermal process,
well as sodium dodecyl benzene sulfonate.
as
[
13]
metal sulfide MoS , one of the famous inorganic
Complexes
2
nanomaterials, was commonly used as catalyst for CO
methanation and hydrodesulfurization. In order to
of metal with citric acid have been reported for a long
[5]
[6]
[14]
time.
an additive.
Because of that, citric acid was widely used as
[
15]
[16]
improve the activity of MoS catalysts, different synthetic
Landschoot,
for example, found that
2
[
7]
methods have been tried, such as calcination method,
the citric acid complex method was an effective way to
[8]
[9]
thermal decomposing,
thermal synthesis
direct sulfurization,
and so forth.
hydro-
prepare submicron doped LiCoVO . Some researchers
4
[10]
also reported that citric acid could facilitate the formation
[
17]
Among the preparation methods, hydrothermal syn-
thesis was considered as an effective way to prepare
of ZnO nanostructure.
In this paper, MoS nanomaterial was synthesized by
2
nanosized MoS due to its mild synthetic conditions and
hydrothermal synthesis route. We chose citric acid as
2
Appl Organometal Chem. 2018;e4339.
wileyonlinelibrary.com/journal/aoc
Copyright © 2018 John Wiley & Sons, Ltd.
1 of 7
https://doi.org/10.1002/aoc.4339

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LI ET AL.
the additive and investigated the effect of citric acid on the
transferred to a sample holder attached to the microscope
in a stream of N2.
catalytic activity of MoS toward sulfur‐resistant metha-
2
nation of syngas. The effect of preparation process on
the MoS2 catalytic activity towards sulfur‐resistant
methanation of syngas was investigated to understand
the structure‐activity relationship.
2.3 | Catalytic evaluation
The catalytic methanation activity of the as‐prepared
MoS (1 ml) was tested in a continuous flow fixed‐bed
reactor. Before reaction, the catalyst was sulfurized in situ
2
2
2
| EXPERIMENT
by a 3% H S/H flow at 400 °C for 4 h. The catalytic activ-
2
2
.1 | Catalyst Preparation
ity test was performed at 3.0 MPa and 500 °C with (3%
H S/H )/CO ratio as 1, N composition as 20 vol.% and
2
2
2
All chemical reagents used were analytic purity without
further purification. In typical synthesis,
Na MoO ⋅2H O (0.02 mol), Na S⋅9H O (0.08 mol) and
–
1
space velocity as 6000 h . After water was condensed by
a
a cold trap, the outlet dry gases containing CO, CH and
4
2
4
2
2
2
CO were analyzed qualitatively with an online SP3420
2
NH OH⋅HCl (0.04 mol) were dissolved in distilled water
2
GC system equipped with a set of thermal conductive
detector (TCD) and a set of flame ionized detector (FID).
The CO methanation performance was represented by
(120 ml). And then citric acid (0.002 mol) was added into
the above solution under vigorous stirring. The mixture
solution was stirred for 30 min. Meanwhile the pH value
was kept at 6 by adding 2 mol/L hydrochloric acid
dropwise. The obtained solution was then transferred into
a 250 ml Teflon‐lined stainless steel autoclave and heated
at 220 °C for 24 h. After the autoclave naturally cooling
down to room temperature, black precipitates were col-
lected by suction filtration then washed with distilled
water and anhydrous ethanol for several times. Finally,
the obtained product was dried under vacuum at 60 °C
for 10 h. The catalysts can be denoted as Cat‐X (X repre-
sented the mole ratio of citric acid and Mo element,
corresponding to the value of 0, 0.1, 0.5 and 1). The Cat‐
the conversion of CO and the selectivity of CH , which
4
were calculated according to the following formulas:
1
catalyst was renamed as Cat‐1‐N when the reductant
NH OH⋅HCl was removed in the catalyst preparation
2
process.
2.2 | Catalyst Characterization
The X‐ray diffraction (XRD) patterns were recorded
in a RigakuD/max‐2500 X‐ray diffractometer, using a Ni‐
filtered Cu‐Kα (λ=1.54056 Å) radiation. The scan speed
o
o
was 8 /min with a scanning angle ranged from 10 to 90 .
IR spectra were obtained on Nicolet6700 FTIR spec-
‐1
trometer with a resolution of 4 cm and KBr was used as
‐1
‐1
the wafer. The scan range was from 4000 cm to 400 cm .
To investigate the morphology of the catalysts, SEM
images were obtained by using a field emission scanning
electron microscopy instrument (FE‐SEM, Nanosem430,
FEI).
The high‐resolution transmission electron microscopy
(HRTEM) was performed using a JEOL JEM‐2010F trans-
mission electron microscope. The catalyst powder was
ultrasonically dispersed in ethanol, and the testing sample
was prepared by dropping the suspension onto a carbon
film supported by a Cu grid. Then the sample was
FIGURE 1 Methanation performance on different catalysts (a:
CO conversion, b: CH selectivity)
4

LI ET AL.
3 of 7
nCOin−n
ðCO;outÞ
hydrochloride acid was removed, CO conversion of Cat‐
XCO ¼
× 100%
n
ðCO;inÞ
1‐N was higher than the Cat‐1 and Cat‐0. It suggested that
n
ðCH4;outÞ
citric acid displayed a positive effect when reductant
SCH ¼
× 100%
4
n
n
þ nðC2H6;outÞ
ðCH4;outÞ
NH OH⋅HCl was removed in catalyst preparation process.
2
n
ðC2H6;outÞ
To understand this phenomenon, lots of characterizations
were carried out to explain the effect of citric acid on the
methanation activity in the following section.
SC H ¼
× 100%
2
6
ðCH4;outÞ
þ nðC2H6;outÞ
ꢀ1
ꢁ
−n
þ n
ðCO;outÞ
þ nðCH4;outÞ þ 2nðC2H6;outÞ
ðCO2;outÞ
CB ¼
×100%
n
ðCO;inÞ
3
.2 | Characterization results and
Here C is the carbon balance after reaction, which is
in the range of ±3% for all the tests, proving the credibility
of the experimental results.
B
discussion
Figure 2(a) was the XRD pattern of the Cat‐0 sample pre-
pared at 220 °C for 24 h without citric acid added. It could
be seen that all peaks were corresponding to the peaks of
3
3
| RESULTS AND DISCUSSION
.1 | CO methanation performance
2H‐MoS , which were in good agreement with the standard
2
card values (JCPDS No. 37‐1492). No peaks related to other
species were detected, indicating that Mo was mainly
The methanation performances of the as‐prepared MoS2
catalysts with different amounts of citric acid were shown
in Figure 1. As showed in Figure 1, the effect of citric acid
on the catalytic methanation activity was negative when
reductant hydroxylamine hydrochloride acid was used.
With the increase of citric acid amount, CO conversion
of Cat‐0.1, Cat‐0.5, Cat‐1 decreased more greatly. When
the mole ratio of citric acid and Mo reached 1, CO
conversion of Cat‐1 was less than 10% after reaction
for 8 h. However, when the reductant hydroxylamine
existed as MoS state in the obtained Cat‐0 sample without
citric acid added. Meanwhile, the peak of (002) was very
sharp and high, indicating that the layered structure of
2
MoS could be well formed with high crystallinity by
2
hydrothermal method. When the citric acid was introduced
during the catalyst preparation process, the peaks corre-
sponding to MoO began to appear. Just as shown in
3
Figure 2(b) for Cat‐0.5 product, the peaks corresponding
to MoS became weak while the peaks corresponding
2
to MoO3 became more intense. With the amount of
FIGURE 2 XRD patterns of the as‐prepared catalysts with different amounts of citric acid added (a: Cat‐0, b: Cat‐0.5, c: Cat‐1, d: Cat‐1‐N)

4
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LI ET AL.
citric acid increased, it could be observed that when
the mole ratio of citric acid and Mo reached 1, all of the
peaks were corresponding to MoO . This illustrated that
3
[14]
the formed molybdenum‐citrate complex
did not react
with sulfur ion to generate MoS but decomposed into
2
MoO under this condition. The MoO could not be fully
3
3
sulfurized by H S under 400 °C. Since it is generally
2
accepted that MoS is the active site for sulfur‐resistant
2
CO methanation process, it is easy to understand why CO
methanation activity became worse with the increasement
of citric acid content.
In complementary with XRD, Figure 3 showed the
FTIR spectra of different catalysts. Just as delivered in
‐
1
‐1
Figure 3(a), for Cat‐1, the peaks at 988 cm , 874 cm and
6
‐1
01 cm were assigned to Mo=O stretch vibration mode,
asymmetric Mo‐O bond and Mo‐O‐Mo bonding vibration
[
18]
mode of MoO , respectively.
1
Besides, the absorption at
3
635 cm‐ was attributed to the asymmetric stretching
vibrations of C=O in carboxyl groups (‐COO ). No peak
at 700‐690 cm was detected which was considered as anti-
symmetric vibrations of the non‐linear Mo‐O‐Mo bridge in
1
‐
[19]
‐1
[14]
dimers,
indicating that molybdenum‐citrate complex
had been decomposed under the hydrothermal condition.
The FTIR spectra was in good consistent with the above
XRD analysis results.
We speculated that hydroxylamine hydrochloride
could inhibit molybdenum‐citrate complex from reacting
with sulfur ion to form MoS species. Therefore, verifica-
2
tion experiment without hydroxylamine hydrochloride
added was conducted and the obtained product of Cat‐1‐
N was analyzed by XRD and FTIR spectra. As observed in
Figure 2(d), the main peaks were corresponding to
FIGURE 3 FTIR spectra of as‐prepared products (a: Cat‐1, b: Cat‐
1‐N)
the peaks of 2H‐MoS , which indicated that molybde-
2
num‐citrate complex had reacted with sulfur ion under
this condition. Sulfur ion substituted the oxygen in
Mo=O bond of the molybdenum‐citrate complex and
the newly generated complex was transformed into
[
5,20]
catalyst.
Therefore, the Cat‐1‐N with rich sulfur
vacancy structure displayed a better methanation perfor-
mance. Another proof could be observed in FTIR spectra.
As depicted in Figure 3 for Cat‐1‐N sample, there was no
peak at 700‐690 cm which was assigned to the antisym-
metric vibration of non‐linear Mo‐O‐Mo bridge in dimers,
indicating that the newly generated sulfur complex had
been decomposed to MoS
To further investigate the morphology and particle
size of the obtained MoS catalyst Cat‐1‐N, SEM image
was also carried out. As shown in Figure 4, the obtained
MoS catalyst exhibited uniform MoS cluster structure
MoS under the hydrothermal condition. By comparing
2
‐
1
the XRD patterns of Cat‐0 (Figure 2a) and Cat‐1‐N
(Figure 2d), we found that Cat‐0 was well crystallized,
while the Cat‐1‐N had a poorly crystallized structure
since peaks for Cat‐1‐N were weaker and more broad-
ened. In addition, calculating the crystal particle size of
.
2
MoS (002) plane by XRD spectra, we found that it was
2
2
5
.6 nm and 2.2 nm for Cat‐0 and Cat‐1‐N, respectively.
It was widely recognized that a decrease in crystallite size
2
2
of MoS (002) plane means a deterioration in crystallinity
with the diameter of about 900 nm. The magnified image
revealed that the cluster was consist of several nanosheets
with the thickness around 20‐30 nm.
2
of the catalyst and decreased MoS stacking layers based
2
[5]
on the same lattice spacing. So, it could be concluded
that Cat‐1‐N had a lower crystallinity and fewer MoS2
stacking layers than the Cat‐0. The poorly crystallized
structure would generate more sulfur vacancies, which
were generally considered as the active sites of MoS2
To better understand the MoS microstructure of Cat‐
2
1 and Cat‐1‐N, HRTEM characterization was also carried
out. As shown in Figure 5(a), the as prepared Cat‐1‐N cat-
alyst had typical multi‐layered MoS structure, while in
2

LI ET AL.
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FIGURE 4 The SEM image of the catalyst Cat‐1‐N
Figure 5(b), the Cat‐1 catalyst contained less MoS phase.
2
In addition, MoS fringes of the Cat‐1‐N catalyst displayed
2
clearly curvature with larger angles, compared to the Cat‐
1
catalyst with highly straight layered structure. It has
been reported that curved nanostructure of MoS catalyst
2
would exhibit a better hydrogenation activity resulting
from the formation of defect sites on the curved basal
[21]
plane.
The result of the TEM images well explained
why the Cat‐1‐N displayed best methanation activity,
which was also consistent with the XRD and FTIR charac-
terization results.
3.3 | Discussion
Based on the above results and discussions, a reaction
network was proposed as described in Figure 6 to further
illustrate the reaction process when the citric acid was
introduced into the synthesis process. Just as shown in
FIGURE 5 The HRTEM images of the catalysts (a: Cat‐1‐N, b:
Cat‐1)
2
‐
Figure 6, firstly, MoO4 would react with citric acid to
form molybdenum‐citrate complex with the
2
‐
increasement of citric acid content while MoO4 without
citric acid would generate MoS with good crystallinity to
When NH OH⋅HCl was removed in the preparation
2
2
obtain Cat‐0. After that, two pathways by adding
process, sulfur ion would more prone to substitute the
oxygen in Mo=O bond of the molybdenum‐citrate com-
plex to form another new complex, which could be
NH OH⋅HCl or not were proposed, which could clearly
2
show the intermediates formation pathways in these
two situation. When NH OH⋅HCl was used, molybde-
decomposed into MoS with poorly crystallized structure
2
2
num‐citrate complex would be prevented to further react
with sulfur ion, which led to the decomposition of
to obtain Cat‐1‐N. So, in this condition, the reaction pro-
cess was carried out by another pathway, which could
be described as follows:
molybdenum‐citrate complex into MoO to obtain Cat‐
3
0
.1, Cat‐0.5, Cat‐1 under hydrothermal synthesis. Under
such conditions, the detailed synthetic reaction could
be formulated as follows:

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LI ET AL.
FIGURE 6 The reaction network on
different catalysts with or without
NH OH⋅HCl added
2
4
| CONCLUSION
CONFLICT OF INTEREST
No conflict of interest exits in the submission of this man-
uscript, and the manuscript is approved by all authors for
publication.
The MoS nanomaterial had be synthesized by a facile
2
hydrothermal route. When citric acid was introduced,
the catalytic activity toward sulfur‐resistant methanation
became worse due to the decomposition of molybde-
num‐citrate complex into MoO3. However, when
ORCID
NH OH⋅HCl was removed, citric acid played a positive
2
Zhenhua Li http://orcid.org/0000-0002-4101-2767
Zijia Yin http://orcid.org/0000-0002-7794-8786
role on the catalytic activity of MoS toward sulfur‐resis-
2
tant methanation. Characterization results proved that
NH OH⋅HCl could effectively inhibited molybdenum‐cit-
2
rate complex to react with sulfur ion to form MoS active
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