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Accepted Manuscript
Novel application of MgH2/MoS2 hydrogen storage materials to
thiophene hydrodesulfurization: A combined experimental and
theoretical case study
Zongying Han, Haipeng Chen, Xinyuan Li, Ruiqian Jiang, Shixue
Zhou
PII:
DOI:
Reference:
S0264-1275(18)30651-8
doi:10.1016/j.matdes.2018.08.036
JMADE 7334
To appear in:
Materials & Design
Received date:
Revised date:
Accepted date:
12 April 2018
15 August 2018
19 August 2018
Please cite this article as: Zongying Han, Haipeng Chen, Xinyuan Li, Ruiqian Jiang,
Shixue Zhou , Novel application of MgH2/MoS2 hydrogen storage materials to thiophene
hydrodesulfurization: A combined experimental and theoretical case study. Jmade (2018),
doi:10.1016/j.matdes.2018.08.036
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ACCEPTED MANUSCRIPT
Novel application of MgH2/MoS2 hydrogen storage materials to thiophene
hydrodesulfurization: a combined experimental and theoretical case study
Zongying Han a, Haipeng Chen b, Xinyuan Li a, Ruiqian Jiang a, Shixue Zhou a,c,*
a
College of Chemical and Environmental Engineering, Shandong University of Science and
b
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Technology, Qingdao 266590, China.
College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang
State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong
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c
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471934, China.
Province and the Ministry of Science and Technology, Shandong University of Science and
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Technology, Qingdao 266590, China.
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*Corresponding author. Email address: zhoushixue66@163.com (S.X. Zhou)
Abstract: In addition to serving as an important energy carrier, hydrogen storage material
also has the potential to be used as an effective solid reducing agent. This paper is concerned
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with the application of MgH2/MoS2 hydrogen storage materials to thiophene desulfurization
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through catalytic transfer hydrogenation. The hydrogen content of the as-prepared
MgH2/MoS2 composites is determined to be 6.15 wt.% with a dehydrogenation peak
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temperature of 402 oC. Taking MgH2 as hydrogen donor, thiophene hydrodesulfurization has
taken place at atmospheric pressure and at the temperature lower than the onset desorption
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temperature, indicating that a coupling effect occurs between MgH2 decomposition and
thiophene hydrogenation. It is further revealed that sulfur removal in thiophene under the
studied condition preferentially proceeds via direct desulfurization (DDS) route. Our density
functional theory (DFT) calculations manifest that energy barriers of the minimum energy
path for thiophene hydrodesulfurization are all less than 1.35 eV. This exploratory case
study demonstrates the feasibility of catalytic transfer hydrogenation using solid-state
hydrogen storage materials.
Keywords: Hydrogen storage materials; Thiophene; Catalytic transfer hydrogenation; DFT
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1. Introduction
Solid-state hydrogen storage materials are capable of storing hydrogen at higher
volumetric densities than compressed and liquid hydrogen, attracting increasingly
researcher?s attention [1]. A great deal of effort has been made on developing
hydrogen-storage systems, including conventional hydrides, complex hydrides, sorbent
systems, and chemical hydrides [2]. The most anticipated application of hydrogen storage
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materials is as a promising lightweight, compact energy carrier [3]. As early as Hannover
Fair 1998, a Siemens Nixdorf laptop computer was demonstrated, which was powered by a
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laboratory PEM fuel cell and a commercial metal hydride tank [4]. Besides, hydrogen
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storage materials demonstrated good prospects for on-board automotive applications in
combination with fuel cells [5]. Moreover, it has also been reported that hydrogen storage
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materials can be used as high-energy solid rocket propellants, emulsion explosives and even
optical sensors [6, 7].
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Theoretically, solid-state hydrogen storage materials, as hydrogen containers, should
retain all its chemical functions. However, to the best of our knowledge, less attention has
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been paid to solid-state hydrogen storage materials as a functional chemical regent [8]. As
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one of the most abundant elements, hydrogen has been used in various industries, including
ammonia production, petroleum processing as well as oil and fat hydrogenation [9].
Generally, gaseous hydrogen molecular should firstly adsorb on catalyst surface and then
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dissociate into atomic hydrogen before interaction with organic compounds, during which a
high energy barrier needs to be overcome. Therefore, these reactions usually were conducted
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under a relatively trenchant condition of elevated temperature or high pressure. It can be
inferred that if the reactant can directly supply atomic hydrogen, the reaction conditionality
can be significantly reduced. Actually, this strategy has been reported as catalytic transfer
hydrogenation and has been successfully applied to various organic hydrogenations [10-12].
In traditional catalytic transfer hydrogenation process, hydrogen is transferred from an
organic gaseous or liquid donor molecule to the targeted organic acceptors [13]. Far less well
known is the possibility of achieving catalytic transfer hydrogenation with inorganic
solid-state hydrogen storage materials as the hydrogen donor. Theoretically, hydrogen
storage materials such as metal hydrides have lower energy barrier to release atomic
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hydrogen than gaseous hydrogen molecule, indicating that they have potential to be
employed for hydrogenation as hydrogen donors. In the present study, MgH2/MoS2
hydrogen storage material was innovatively applied to thiophene hydrodesulfurization as a
case study, and the possible reaction mechanism was symmetrically investigated. This
exploratory attempt is expected to pave a new direction for application of solid-state
hydrogen storage materials.
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2. Experimental
2.1 Materials preparation and thiophene hydrogenation process
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The Magnesium used for the preparation of hydrogen storage materials had a purity
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of >99.5wt% (Tianjin Kermel Chemical Reagent Co., Ltd, China) and the MoS2 used as
lubricant had a purity of >98.5%. The argon used as ball-milling atmosphere has a purity
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of >99.9 vol.% (Qingdao Hengxiang Gas Company, China). The hydrogen used for static
hydrogenation process has a purity of >99.9 vol.% (Qingdao Hengxiang Gas Company,
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China). The thiophene has a purity of >99.0 wt.% (Chengdu Kelong Chemical Company,
China).
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Ball milling to prepare hydrogen storage materials was carried out on a planetary
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ball-mill with (ND7-2 model, Nanda Tianzun Instrument Company, China) four stainless
steel vials of 250 mL with stainless steel balls. The work revolution of the main axis of the
mill was set at 270 r/min and a reverse direction period of 10 min. Each vessel was charged
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with 10 g raw materials contained Mg and MoS2 in weight ratio of 9:1, and the
ball-to-powder weight ratio (BPR) was about 48:1 except for the research on the effect of
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BPR. The vessels were purged with argon before the ball milling. The milling time was
normally set at 3 h except for the research on the effect of milling time. After milling, the
Mg/MoS2 hydrogen storage materials were displaced in a glove box with argon atmosphere
to prevent the materials from being oxidized. Then all the static hydrogenation process was
performed in a tubular reactor under 2 MPa H2 at 300 oC.
The thiophene hydrodesulfurization was carried out in a fixed bed reactor system
operating in the dynamic mode at atmospheric pressure at varied temperatures ranging from
325 to 400 oC. Initially, 2 g of MgH2/MoS2 composites were charged into the reactor and
thiophene gas with 10 g/m3 (20 oC, 0.1 MPa) carried by argon was introduced into the
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reactor at a flow rate of 500 mL/min from a gas cylinder. Then, the tubular reactor was
placed into a tubular resistance furnace at a specified temperature to initiate thiophene
hydrogenation reaction. Gaseous products generated by the reaction were firstly introduced
into an absorption bottle containing Pb(NO3)2 solution for the absorption and qualitative
detection of H2S, and then introduced into an absorption bottle containing ethanol solution to
absorb other gas products and unreacted thiophene.
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2.2 Characterization methods of the materials
The micro-morphology observation of the as-prepared hydrogen storage materials was
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performed on ZEISS MERLIN Compact and JSM-7800F Field Emission Scanning Electron
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Microscopes (SEM) equipped with an Oxford X-MAX energy dispersion spectrometry
(EDS). The TEM analysis was conducted on a FEI Tecnai G2 F20 transmission electron
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microscope. Particle size distribution analysis was performed on Malvern Zetasizer Nano
and Malvern Mastersizer 2000 particle size analysers with ethanol as a dispersant. The
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crystal structure of the materials was determined by a Bruker D8 Advance X-ray
diffractometer (XRD) in stepsize of 0.02o operated at 40 kV and 150 mA with Cu K?
radiation. The thermoanalysis of the materials was carried out on Netzsch STA 449C
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simultaneous TG-DSC thermoanalyzer or Mettler-Toledo DSC1 instrument at a heating rate
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of 10 oC/min and an argon flow rate of 60-100 mL/min. The measurement of hydrogen
content was performed at the pressure of 0.1 MPa using an apparatus designed according to
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water displacement method. The gas chromatography (GC) analysis of the gas product from
thiophene hydrogenation was performed on a Shimadzu GC-2010 Plus chromatograph with
mm� um).
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a flame ionization detector (FID) and a capillary column of RT@-Q-BOND (30 m�53
2.3 Computational models and methods
All of the calculations in this work were carried out with the periodic density functional theory
package DMol3 in Materials Studio (Accelrys Software Inc.). The exchange and correlation
energies are employed using the Perdew-Burke-Ernzerhof (PBE) functional within the generalized
gradient approximation (GGA) [14]. The calculated equilibrium lattice constant of bulk MgH2 is c/a
= 0.672, which is close to the value reported in previous theoretical and experimental studies (Table
1). The cohesive energy of MgH2 is calculated to be 12.9 eV, which is in line with the value of
11.2-13.5 eV reported in previous study. An isolated thiophene molecule has been optimized in a
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cubic box of 20 � side to obtain its equilibrium properties. The bond lengths and bond angles in
thiophene molecular are in good agreement with the experimental and theoretical values (Table 2).
The results obtained in these tests confirmed the reliability of the bulk model and made us confident
in pursuing the next step of our investigations. A seven-layered MgH2 (001) slab was constructed
by using (3� super-cells containing Mg63H126 and vacuum space (15 �) as periodic boundary
conditions (Fig. 1). It should be noted that this setting has been carefully checked changing the
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thickness of the vacuum layer and the number of atom layers (Fig. S1). The increase in thickness of
the vacuum layer was found to change the binding energy of a thiophene molecule by less than
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0.001 eV/�, and especially, the surface formation energy dependence on the size of the number of
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atom layers was less than 0.0001 eV. To simulate the MoS2-modified MgH2 model, MoS2/MgH2
(001) surface containing a small MoS2 chain (one Mo and two S atoms) over the above mentioned
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p(3� MgH2 substrate was constructed (Fig. 1). This model is carefully evaluated and selected
from 15 alternative models through balancing the model representation and accuracy as well as the
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computing cost (Table S1). The theoretical MoS2 loading content (1.59 mol.%) is very close to the
experimental value (1.69 mol.%). Subsequent calculations for MgH2 (001) surface were performed
with a 2?2?1 k-point mesh in the Monkhorste Pack scheme [15]. The dependence of total energy
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on k-point was also checked. The difference of total slab energy is less than 0.001 eV when k-point
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was higher than 2 (Fig. S1). Thus, the 2 Monckhorst-Pack k-point grid was used to limit
the computing costs. The topmost three layers and the adsorbents were allowed to relax, while the
positions of the rest atoms were fixed. The geometry optimization calculations were achieved with a
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convergence tolerance in maximum energy change of 2.7212?10-4 eV (2.6255?10-2 kJ/mol),
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maximum force of 0.054424 eV/� (5.251 kJ/mol/�) and maximum displacement of 0.005 �,
respectively. Moreover, the transition states and energy barriers are determined by means of
complete LST/QST method for reactions [16]. The convergence criterion for transition state
calculations was set to a root-mean-square force on atoms tolerance of 0.27212 eV/� (26.255
kJ/mol/�).
Table 1 The calculated equilibrium lattice constant and cohesive energy of MgH2.
Parameters
This work
Experimental [17, 18]
Theoretical [18, 19]
a/�
4.5331
4.5168
4.495
c/�
3.0443
3.0205
3.002
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c/a
0.672
0.669
0.668
Ecohesive/eV
12.9
13.4
11.2-13.5
Table 2 Geometric parameters for the optimized isolated thiophene derived from our DFT
calculations.
C1-C2
C2-C3
?C-S-C
?S-C-C
This work
1.728 �
1.376 �
1.423 �
91.81�
111.45�
Experimental [20]
1.714 �
1.370 �
1.419 �
92.2�
111.4�
Theoretical [21]
1.725 �
1.364 �
1.424 �
91.7�
111.4�
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The adsorption energy ???? is defined as follows:
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C-S
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???? = ??????????/???? ? ?????????? ? ?????
(1)
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where ??????????/???? is the total energy of the slab together with the adsorbate, ?????????? is the
total energy of the isolated adsorbate obtained in a 20�� � cubic cell, and ????? is the total
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energy of the bare slab.
For an elementary reaction or diffusion process, the energy barrier ???? and reaction heat ??
are calculated on the basis of the following formulas:
(2)
?? = ??? ? ???
(3)
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???? = ??? ? ???
where ??? , ??? and ??? represent the total energy of the initial state reactants, the transition
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states and the final state products, respectively.
3. Results and discussion
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3.1 Phase structure, morphology and hydrogen sorption properties
In order to check the effect of milling time on the crystal structure, a series of Mg/MoS2
composites was prepared by milling from 0 to 4.0 h. According to the Scherrer equation [22],
the crystallite size of Mg was calculated from XRD patterns (Fig. S2). It was found that the
crystallite size of Mg decreased by 19.4 nm after milling for 2 h, and further reduced with a
gradually decreasing rate when extending milling time. For milling from 3 h to 4 h, the
crystallite size of Mg only decreased by 1.4 nm, indicating that the crystallite size of Mg
would remain constant with the subsequent milling process. This is mainly attributed to the
lubricating effect of MoS2 by weakening the abrasive force, which has been demonstrated in
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many previous studies [23-25]. This can also be evidenced through particle size distribution
analysis (Fig. S3). It was found that the average particle size of Mg/MoS2 composites was
increased from 144 ?m to 208 ?m after 2.0 h milling. The average particle size kept almost
unchanged (207 ?m) when further milling to 3.0 h. This means that the presence of MoS2
can successfully prevent particle agglomeration induced by cold welding. In addition, the
effect of BPR on the crystal structure of Mg/MoS2 composites was also investigated (Fig.
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S4). It was found that the crystallite size of Mg was reduced by only 0.7 nm after milling 3.0
h at low PBR of 12:1. At higher PBR of 24:1 and 48:1, the reductions of crystallite size are
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11.1 nm and 25.4 nm respectively. Increased PBR is expected to introduce more strain and
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increase the defect concentration in the powder and thus lead to easier amorphization. Based
on above findings, the Mg/MoS2 composite prepared by milling 3.0 h at PBR of 48:1 was
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carefully selected for the subsequent investigation.
Fig. 2 shows XRD patterns of the as-prepared MgH2/MoS2 hydrogen storage materials.
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It can be seen that the diffraction peaks of Mg are still very strong and sharp after 3h of
milling (Fig. 2a), indicating a good crystallinity of Mg. This can be directly reflected in the
TEM selected-area electron diffraction (SAED) patterns as shown in Fig. 3c, where both
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diffraction spot and diffraction ring of Mg were obviously observed. In the HRTEM image,
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the Mg phase was found in the examined zone by match the interplanar spacing of 0.24 nm
with Mg (101) lattice spacing (PDF#75-7195). After static hydrogenation process at 300 oC,
the main phase transformed from Mg into MgH2 through reaction Mg + H2 ? MgH2 , of
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which ?rSo=-130.5 J/(K穖ol H2) and ?rGo=-39.6 kJ/mol H2 at 273 K. The hydrogen content
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was measured to be 6.15 wt.%, which means about 88.2% of Mg was hydrogenated. From
the XRD pattern in Fig. 2b, the crystallite size of MgH2 was calculated to be about 52.6 nm.
The HRTEM images of MgH2/MoS2 composite powders obtained after static hydrogenation
is shown in Fig. 3e. The examined zones of MgH2 matrix showed the existence of ?-MgH2,
as confirmed by the interplanar spacing of 0.22 nm agreeing with the (200) lattice spacing of
tetragonal ?-MgH2 (PDF#74-0934). Fig. 2c shows the XRD pattern of MgH2/MoS2
hydrogen storage materials after hydrogen desorption, and Mg re-become the dominant
phase as initially. The existence of Mg can also be proved by HRTEM image and SAED
pattern shown in Fig. 3h and Fig. 3i. Interplanar spacing of 0.28 nm was observed in the
examined zone, which is corresponding that of Mg (100) lattice spacing. Some strong spots
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and continuous rings were well matched with several lattices spacing of Mg, agreeing with
the XRD data. The mutual transformation between MgH2 and Mg demonstrated the
reversibility of MgH2/MoS2 hydrogen storage materials. The observed existence of MgO in
XRD and SAED patterns are originated from sample exposure to air during testing process.
In addition, no contribution from MoS2 phases was observed in the XRD patterns in Fig.
2, testifying that MoS2 existed in the materials in the form of amorphous phase. This can be
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evidenced by the XRD observation (Fig. S5). The distribution of Mo and S elements can be
viewed in Fig. 4 detected by EDS mapping. Benefiting from ball milling, MoS2 uniformly
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dispersed on the surface of Mg particles (Fig. 4a). It can be seen from the element mapping
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image that the distributions of S and Mo elements are the same. Since the mole fraction of
MoS2 loading on Mg is only 1.69 mol.%, the distribution counts of Mo and S elements are
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sparsely scattered compared to that of Mg element. After hydrogenation, Mo and S elements
still uniformly dispersed on the MgH2 particles (Fig. 4b). This indicates that there is no
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particle aggregation of MoS2 during the hydrogenation process. The impurity SiO2 was
introduced from the as-received MoS2 powder, which showed strong diffraction peak of
quartz through XRD determination (Fig. S5).
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Fig. 5 shows the morphology and particle size distribution of the MgH2/MoS2 hydrogen
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storage materials. After ball milling for 3.0 h, Mg particles exhibit defective bumpy surface
(Fig. 5d), which is beneficial to the subsequent hydrogenation. As can be seen in Fig. 5g,
there is a fairly tight monomodal particle size distribution peaked at a particle diameter of
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approximate 224 ?m, and the particle size span is determined to be 1.401. The MoS2
additive is not discernible in the micrograph, indicating that it is homogeneously dispersed
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on the Mg particles. After hydrogenation, the average particle size is reduced to 162 ?m
while the particle size span increased to 2.581 (Fig. 5h). This is mainly attributed to phase
transformation induced particles split. During hydrogenation, transformation from Mg to
MgH2 will lead to lattice distortion and volume expansion, producing a lot of internal stress
inside particles. Therefore, particles will rupture to release the internal stress. As evidenced
in Fig. 5e, the transformation-induced cracks were observed in the particles. It was reported
in previous study that more active H atoms might exist in the crack gaps on the surface of
MgH2 crystallites [26], and this might be beneficial to the subsequent thiophene
hydrodesulfurization. The average particle size is further reduced to 125 ?m and particle size
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span further increased to 3.440 after dehydrogenation (Fig. 5i). Obviously, a double-modal
size distribution is presented peaked at 7 ?m and 200 ?m respectively. The
transformation-induced cracks still existed and remained unhealed after dehydrogenation as
can be seen in Fig. 5f.
Fig. 6 shows the DSC curves of the hydrogenated MgH2/MoS2 hydrogen storage
materials. It can be seen that the endothermic peak of MgH2/MoS2 composites was quite
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sharp with desorption temperature range of 391-420 oC. The derived desorption peak
temperature was 402 oC, which was lower than that of commercial pure MgH2 (451 oC). The
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effect of MoS2 content on the desorption temperature of MgH2/MoS2 hydrogen storage
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materials were preliminary investigated (Fig. S6). It was found that the dehydrogenation
temperature of MgH2/MoS2 hydrogen storage materials increased with the content of MoS2.
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This was mainly due to the less crystallite size reduction of Mg caused by the lubrication of
more MoS2, corresponding to larger crystallite size of MgH2.
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3.2 Thiophene hydrodesulfurization
Traditionally, thiophene hydrodesulfurization was carried out with gaseous hydrogen as
the hydrogen donor. All reactants in the heterogeneous reaction were gas phase, and
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therefore high pressure was required to promote the reaction [27]. Thus, using solid state
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hydrogen storage materials to replace gaseous hydrogen as hydrogen donor is expected to
facilitate the thiophene hydrodesulfurization under a less critical condition. In the present
study, thiophene hydrodesulfurization with MgH2/MoS2 hydrogen storage material was
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carried out at temperature ranging from 325 to 400 oC at atmospheric pressure. The reactions
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were evaluated by the products in the solid phase and gas phases which were determined by
XRD and GC analysis respectively.
Fig. 7 shows the gas chromatograms of the gas product collected during the thiophene
hydrogenation reaction at different temperature. It can be seen that thiophene hydrogenation
occurred at 325 oC under atmospheric pressure, indicating that thiophene can react with
MgH2/MoS2 hydrogen storage material under mild conditions without high pressure gaseous
hydrogen and precious catalysts. Apart from unreacted thiophene, the main gas phase
products detected by GC are C1-4 alkanes, C2-4 alkenes, butadiene, dihydrothiophene (DHT)
and
tetrahydrothiophene
(THT).
The
gas
phase
composition
of
thiophene
hydrodesulfurization products was determined by comparing with the chromatogram of
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corresponding standard gases. Notably, peaks of DHT or THT were theoretically deduced
due to the limited availability of the standard gases. According to our measurements, there is
a linear relationship between the retention time of thiophene hydrodesulfurization products
on the GC column and boiling point or molecular mass (Fig. S7). This finding is consistent
with that reported in many previous studies [28-30]. Thus, the last peak behind thiophene is
believed to be from DHT or THT. This strategy of identifying the attribution of retention
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peaks has been proven to be helpful without some specific standard gases for composition
analysis [31-33]. Since thiophene is the only carbon source, the above carbon-containing
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products must originate from thiophene desulfurization and subsequent thermal cracking
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[34]. It can be inferred that the most likely sulfur-containing product is H2S after cracking.
However, no characteristic peak of H2S was observed because FID detector is nonspecific
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for H2S.
In order to detect the existence of H2S, a simple chemical reaction test was employed
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for the qualitative detection, by which the gas products were introduced into an absorption
bottle containing Pb(NO3)2 solution. Fig. 8a shows the XRD pattern of the precipitate
formed after the gas products were absorbed by Pb(NO3)2 solution. The diffraction peaks of
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the as-obtained precipitate matched well with PbS phase, confirming that the gas products of
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thiophene desulfurization also contained H2S. This proved once again that sulfur in
thiophene can be removed in the form of gaseous H2S at low temperatures under
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atmospheric pressure.
Fig. 8b shows the XRD pattern of the solid material after hydrogenation reaction at 350
C. It can be seen that the strong and sharp diffraction peaks of Mg appeared, meaning that
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o
part of MgH2 decomposed at 350 oC. Notably, the desorption temperature of MgH2/MoS2
composites ranged from 391 to 420
o
C (Fig. 6), and there should not have been
decomposition taking place below the onset desorption temperature. However, the results of
GC show that thiophene has been hydro-decomposed at 325-375 oC. This indicates that a
coupling effect might have occurred between the decomposition of MgH 2 and the
hydrogenation of thiophene. Similar findings were also reported in previous literatures
[35-38]. In the study of Yao et al. [35], the effects of MgH2 and Mg(BH4)2 hydrogen storage
materials on the thermal decomposition behaviors of 2,4,6-Trinitrotoluene (TNT) were
investigated. It was found that addition of MgH2 or Mg(BH4)2 to TNT has lowered the onset
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temperature and reduced the apparent activation energies of TNT decomposition. Liu et al.
[36] explored the effects of three Mg-based hydrogen storage materials (MgH2, Mg2NiH4
and Mg2CuH3) on the thermal decomposition of ammonium perchlorate (AP)-based
composite solid propellant. The experimental results indicated that the thermal
decomposition peak temperature of the propellant was decreased and the burning rates
increased using Mg-based hydrogen storage materials as promoter. It was supposed that
the thermal decomposition of the AP-based solid propellant.
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these improvements were attributed to the more complicated reactions and reciprocities in
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In the present study, the coupling effect between the decomposition of MgH2 and the
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hydrogenation of thiophene can be proved by the thermodynamic calculation as shown in
Fig. 9. It can be seen that decomposition of MgH2 is obviously a strong endothermic process
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(?H>0) while the hydrogenation of thiophene is an exothermic process (?H<0). It can be
inferred that the released heat of thiophene hydrogenation can contribute to the required heat
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of MgH2 decomposition. As shown in Fig. 9, the thiophene hydrogenation coupling with
MgH2 decomposition is still an exothermic process (?H<0). Simultaneously, hydrogen
released from MgH2 satisfies the hydrogen demand of thiophene hydrogenation. Therefore,
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the reciprocal heat transfer and hydrogen transfer facility the reaction at lower temperature.
Selective
bond
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3.3 Reaction routes and possible mechanism of thiophene hydrodesulfurization
breaking
determines
the
reaction
route
of
thiophene
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hydrodesulfurization. The average bonding enthalpies of C-S, C-C and C=C are 2.68 eV
(259 kJ/mol), 3.61 eV (348 kJ/mol) and 6.36 eV (614 kJ/mol) respectively [39], indicating
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that the C-S bond scission reaction is preferred. However, it has been proven that conjugated
C=C bond is more readily to be hydrogenated than isolated C=C bond [40, 41]. This is
attributed to the fact that the entire ?-system of conjugated C=C bond is involved in
adsorption through di-?-coordination, which is more favored than the di-? mode of
adsorption of a single double bond [42]. Therefore, the hydrogenation of conjugated C=C
bond in thiophene is also likely to occur. Hence, thiophene hydrodesulfurization reaction
may follow direct desulfurization (DDS) route or hydrogenation desulfurization (HYD)
route (Fig. 10). According to the DDS route, thiophene was firstly hydrocracked into H2S
and butadiene, and further into C2-4 alkenes and C1-4 alkanes. For the HYD route, thiophene
was firstly hydrogenated to form DHT and THT, and further hydrocracked into H2S and C1-4
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alkanes. That which reaction prefers to take place depends on the specific reaction
conditions, including the hydrogen donor, catalyst, temperature and pressure [43, 44].
In the present study, thiophene hydrodesulfurization reaction is believed to occur as a
multi-step reaction including partially hydrogenated intermediates. However, the mechanism
still remains unclear due to limited experimental information. To gain insight into the
mechanism of thiophene hydrodesulfurization using solid state hydrogen storage materials,
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we constructed models for DFT calculations and conducted an extensive investigation. Fig.
11 compares the calculated hydrogen release and diffusion energies from different hydrogen
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donor. According to our DFT calculations, the energy barrier of a single H atom overflow
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from clean MgH2 (001) surface into vacuum is approximately 1.34 eV, which is much lower
than that of H-H bond dissociation in gaseous hydrogen (4.55 eV). These calculated values
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are close to that reported in previous studies [45-47]. This lower desorption energy manifests
that solid state MgH2 hydrogen storage material is more readily to release atomic hydrogen
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than gaseous hydrogen. The hydrogen desorption energy is further reduced to 0.81 eV due to
the catalytic effect of MoS2 loading (Fig. 11a). This theoretical result is in good agreement
with the experimental findings reported in literature [48]. In addition, the energy barriers of
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hydrogen diffusion inside the bulk MgH2 (001) surface was also evaluated. Limited by the
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as-constructed MgH2 surface model, only the hydrogen diffusion processes from the third
inner layer to the second inner layer (3rd to 2nd) and the second inner layer to the first layer
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(2nd to 1st) were investigated (Fig. S8). The activation barriers computed for hydrogen
diffusion from the sub-layers into surface are all less than 1.0 eV, which are also much lower
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than that of dissociation of gaseous H2. Obviously, the whole process combined hydrogen
diffusion and desorption is endothermic, which is consistent with the thermodynamic
prediction (Fig. 9).
In order to examine the mechanism of thiophene hydrodesulfurization on MgH2 (001)
surface, the interaction between thiophene molecule and MgH2 substrate was firstly explored.
A total of 9 different geometries of thiophene adsorption on MoS2-modified MgH2 (001)
surface were examined (Fig. S9 and Table S2). From this preliminary study, it was found
that the most favorable adsorption mode on the MoS 2-modified MgH2 (001) surface was that
the thiophene molecule lied on the MgH2 surface in almost parallel with the S atom slightly
deviating from the surface (Fig. S9). The interaction energy can be decomposed in three
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terms: deformation of the thiophene molecule (-0.02 eV), deformation of the MoS2-modified
MgH2 (001) surface (-0.20 eV), and adsorption energy between the thiophene molecule and
the MoS2-modified MgH2 (001) surface (-0.04 eV). This demonstrates that the interaction
between the thiophene molecule and the surface is actually not high (-0.26 eV), which
means it belongs to a physical process. Most of the other adsorption configurations are
slightly endothermic (-0.01-0.27 eV). Charge population analysis on each atom indicated
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little electron transfer between the thiophene molecule and the surface. The similar
phenomenon was also found in the thiophene adsorption on other catalysts [43, 49, 50].
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Table 3 lists the activation barriers and energy change of elementary steps involved in
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thiophene hydrodesulfurization on MgH2 (001) surface. According to the experimental result,
we investigated the energy changes following both the DDS and HYD routes. Energetically,
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DDS is determined to be the most plausible reaction route (S1?S2?S3?S4) for thiophene
desulfurization on MoS2-modified MgH2 (001) surface as shown in Fig. 11, which can be
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described as follows: one C atom adjacent to S atom in adsorbed thiophene is
pre-hydrogenated with an energy barrier of 0.93 eV (frequency: -983.07 cm-1), which
releases energy of -1.44 eV and weakens the C-S bond. Then, another surface H atom sticks
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to the S atom (reaction barrier: 1.11 eV, frequency: -749.79 cm-1), which leads to C-S bond
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cleavage and surface carbon chain forming. Similarly, a surface H atom attacks another C
atom adjacent to the S atom to weaken the rest C-S bond (reaction barrier: 1.25 eV,
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frequency: -987.61 cm-1). Finally, the S tail of the carbon chain reacts with surface H atom
with a reaction barrier of 1.31 eV, which leads to the second C-S bond scission and releases
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surface H2S molecule. The desorption of H2S molecule needs to overcome an energy barrier
of 0.58 eV. Following the HYD route (S1?S2?S5?S6?S7?S8?S9?S10), it can be
found that the activation energies of two C-S bonds scission (2.85 eV and 2.90 eV
respectively) are much higher than that in DDS route. This indicates that thiophene
hydrodesulfurization under the studied conditions may preferentially proceed via direct DDS
route. Besides, the high stability of DHT and THT on MoS2-modified MgH2 (001) surface
also indirectly proved their existence in the exhaust gas of thiophene hydrodesulfurization
detected by GC in Fig. 7.
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Table 3 A summary of the elementary reactions involved in thiophene hydrodesulfurization
on clean and MoS2-modified MgH2 (001) surface including the activation barriers (???? )
and energy change (??) of the reactions.
Clean
MoS2-modified
Reaction equations
?? (eV)
???? (eV)
?? (eV)
S1
0.93
-1.37
0.93
-1.44
S2
1.21
-2.19
1.11
-2.21
S3
1.26
-1.79
1.25
-1.88
S4
1.19
-3.07
1.31
-3.05
S5
0.70
0.52
-3.13
S6
1.28
-1.21
1.32
-1.40
S7
1.02
-4.05
1.19
-3.87
2.79
-1.16
2.85
-1.18
0.32
-3.52
0.26
-3.52
2.96
-2.05
2.90
-2.05
0.42
-3.32
0.44
-3.30
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S9
S10
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S11
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-3.09
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S8
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???? (eV)
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Steps
For comparison, the elementary reactions involved in thiophene hydrodesulfurization
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on clean and MoS2-modified MgH2 (001) surface was also computed. It can be seen from
Fig. 12 and Table 3 that both energy barrier and reaction heat are similar to that of thiophene
desulfurization on amorphous MoS2-modified MgH2 (001) surface. This indicates that the
amorphous MoS2 loading on MgH2 surface functions as more lubricant or dispersant than
catalyst. This is mainly attributed to two perspectives. Firstly, the content of MoS2 is much
lower than that in traditional MoS2 catalyzed thiophene hydrodesulfurization where MoS2
provide the reaction sites. In the present study, thiophene desulfurization mainly proceeds on
MgH2 surface (the dominant phase) rather than MoS2. Besides, the amorphous MoS2 used in
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the present study does not have the specific structures of MoS2 crystals which can provide
sufficient active sites at the edge [51, 52].
It has been reported that C-S bond cleavage is rate-limiting for thiophene
hydrodesulfurization on most of the transition metal sulfide catalysts [53]. In the previous
DFT study of thiophene hydrodesulfurization over MoS2 catalysts, it was found that both
S-C scission and H2S desorption could be the crucial step in S removal [43]. By comparing
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the activation energies of surface reactions and H2S desorption in the current study, it can be
deduced that the rate-limiting step is C-S bond scission or C=C bond pre-hydrogenation for
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thiophene hydrodesulfurization on MoS2-modified MgH2 (001) surface. It can also be seen
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in Table 3 that all the surface hydrogenation steps are exothermic reaction. This strong
exothermic of thiophene hydrodesulfurization based on DFT calculations is consistent with
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the results predicted by thermodynamic calculations (Fig. 9). Therefore, thiophene
desulfurization over MoS2-modified MgH2 surface through catalytic transfer hydrogenation
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is both thermodynamically allowed and kinetically favored.
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4. Conclusions
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In this study, MgH2/MoS2 hydrogen storage material was prepared by ball milling and
static hydrogenation, and its application to thiophene hydrodesulfurization was investigated
based on experimental and theoretical verifications. The hydrogen content of the composites
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is determined to be 6.15 wt.% with a dehydrogenation peak temperature of 402 oC. Atomic
hydrogen released from MgH2 can react with thiophene at atmospheric pressure and in
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temperature range of 325-400 oC under the catalysis of MoS2. Coupling effect occurs
between MgH2 decomposition and thiophene hydrogenation. The hydrodesulfurization of
thiophene under the studied conditions preferentially proceeds via DDS route with the
highest energy barrier of less than 1.35 eV. This study demonstrates the possibility and
feasibility of thiophene desulfurization using solid state hydrogen storage as hydrogen
donor.
Acknowledgements
The authors wish to acknowledge the financial support from the National Natural
Science Foundation of China (Grant Nos. U1610103 and 21176145), Shandong provincial
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Natural Science Foundation (Grant No. ZR2018BB069), China Postdoctoral Science
Foundation (Grant No. 2018M632692), SDUST Research Fund (Grant No. 2014TDJH105)
and Shenzhen Supercomputer Center.
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this
time as the data also forms part of an ongoing study.
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Figure Captions
Fig. 1 Schematic diagram of bulk MgH2, bulk thiophene, clean MgH2 (001) surface, and
MoS2-modified MgH2 (001) surface models.
Fig. 2 XRD patterns of the MgH2/MoS2 hydrogen storage materials. (a) as-prepared
Mg/MoS2 composites; (b) hydrogenated MgH2/MoS2 composites; (c) de-hydrogenated
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Mg/MoS2 composites.
Fig. 3 Low-magnification and high-magnification TEM images as well as the SAED
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patterns of the MgH2/MoS2 hydrogen storage materials. (a, b and c ) Mg/MoS2 composite
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particles; (d, e and f) MgH2/MoS2 composite particles; (g, h and i) de-hydrogenated
Mg/MoS2 composites.
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Fig. 4 Element distribution in the as-prepared Mg/MoS2 (a) and MgH2/MoS2 (b) composites.
Fig. 5 Low-magnification and high-magnification SEM images and particle size distribution
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curves of the MgH2/MoS2 hydrogen storage materials. (a, d and g ) Mg/MoS2 composite
particles; (b, e and h) MgH2/MoS2 composite particles; (c, f and i) de-hydrogenated
Mg/MoS2 composites.
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Fig. 6 DSC curves of the as-prepared MgH2/MoS2 composites and commercial pure MgH2.
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Fig. 7 Gas chromatograms of the gas product collected during the hydrogenation reaction.
Fig. 8 XRD patterns of the precipitate formed after the sulfur-containing gas products was
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absorbed by Pb(NO3)2 solution (a) and the solid material after hydrogenation reaction at 350
C (b).
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Fig. 9 Calculated enthalpy changes of MgH2 decomposition, thiophene hydrogenation and
their coupling reaction. Both hydrogenation desulfurization (a) pathway and direct
desulfurization (b) pathway are considered. The thermodynamic calculations are performed
using HSC Chemistry software.
Fig. 10 Proposed reaction networks of thiophene hydrodesulfurization with MgH2.
Fig. 11 Calculated H release (a) and diffusion (b) energies from pure and MoS2-modified
MgH2 surfaces.
Fig. 12 Energy profile of the minimum energy path for thiophene hydrodesulfurization on
clean MgH2 (001) surface (a) and MoS2-modified MgH2 (001) surface (b).
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Fig. 1 Schematic diagram of bulk MgH2, isolated thiophene, clean MgH2 (001) surface, and
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MoS2-modified MgH2 (001) surface models.
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Fig. 2 XRD patterns of the MgH2/MoS2 hydrogen storage materials. (a) as-prepared Mg/MoS2
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composites; (b) hydrogenated MgH2/MoS2 composites; (c) de-hydrogenated Mg/MoS2 composites.
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Fig. 3 Low-magnification and high-magnification TEM images as well as the SAED
patterns of the MgH2/MoS2 hydrogen storage materials. (a, b and c ) Mg/MoS2 composite
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particles; (d, e and f) MgH2/MoS2 composite particles; (g, h and i) de-hydrogenated
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Mg/MoS2 composites.
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Fig. 4 Element distribution in the as-prepared Mg/MoS2 (a) and MgH2/MoS2 (b) composites.
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Fig. 5 Low-magnification and high-magnification SEM images and particle size distribution
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curves of the MgH2/MoS2 hydrogen storage materials. (a, d and g ) Mg/MoS2 composite
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Mg/MoS2 composites.
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particles; (b, e and h) MgH2/MoS2 composite particles; (c, f and i) de-hydrogenated
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Fig. 6 DSC curves of the as-prepared MgH2/MoS2 composites and commercial pure MgH2.
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Fig. 7 Gas chromatograms of the gas product collected during the hydrogenation reaction.
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Fig. 8 XRD patterns of the precipitate formed after the sulfur-containing gas products was
absorbed by Pb(NO3)2 solution (a) and the solid material after hydrogenation reaction at 350
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Fig. 9 Calculated enthalpy changes of MgH2 decomposition, thiophene hydrogenation and
their coupling reaction. Both hydrogenation desulfurization (a) pathway and direct
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desulfurization (b) pathway are considered. The thermodynamic calculations are performed
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using HSC Chemistry software.
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Fig. 10 Proposed reaction networks of thiophene hydrodesulfurization with MgH2.
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Fig. 11 Calculated H release (a) and diffusion (b) energies from clean and MoS2-modified
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Fig. 12 Energy profile of the minimum energy path for thiophene hydrodesulfurization on
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Author Contributions
Shixue Zhou conceived of the hypothesis. Zongying Han performed the experiments and DFT
calculations, and wrote the manuscript after discussions the results with all authors. Haipeng
Chen gave a lot of valuable comments on the data analysis. Xinyuan Li and Ruiqian Jiang
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contributed on SEM, DSC and XRD analysis.
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Graphical Abstract
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Highlights
? Solid-state hydrogen storage materials are employed for catalytic transfer hydrogenation.
? Coupling effect occurs between MgH2 decomposition and thiophene hydrogenation.
? Energy barriers in the minimum energy path for thiophene hydrodesulfurization are less than
1.35 eV.
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? Thiophene desulfurization with MgH2 is both thermodynamically allowed and kinetically
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favored.
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