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Revealing the dynamic formation mechanism of porous Mo 2 C: an in-situ TEM study

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In-situ transmission electron microscopy (TEM) enables direct observation of the micromorphology and microstructure evolution of catalysts in the chemical atmosphere. Studying the structural evolution during the formation of molybdenum carbide using in-situ TEM is helpful for the preparation of high-performance carbide catalysts. Herein, the formation mechanism of porous Mo2C from MoO2 nanoparticles (NPs) was studied by in-situ TEM. The formation of Mo2C was induced by the defects of MoO2, and the formed Mo2C facilitated the carbonization of neighboring MoO2 NPs. The growth rate of Mo2C between MoO2 NPs was slower compared to that within a single MoO2 NP. In addition, the formation and growth of pores in Mo2C were also studied; the pores grew radially during the early stages from the nucleation sites and later grew branched and curved. As Mo2C underwent competitive growth, the pores transitioned from straight to curved. Eventually, during prolonged carbonization at high temperatures, Mo2C underwent sintering.
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Wang et al. Chem Synth 2023;3:42
DOI: 10.20517/cs.2023.33 Chemical Synthesis
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Open AccessResearch Article
Revealing the dynamic formation mechanism of
porous Mo2C: an in-situ TEM study
Yongzhao Wang1,2,#, Yiming Niu1,2,# , Yinghui Pu1,2, Shiyan Li3, Yuefeng Liu3 , Bingsen Zhang1,2,*
1Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang
110016, Liaoning, China.
2School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, Liaoning, China.
3Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
Liaoning, China.
#The authors contributed equally to this work.
*Correspondence to: Prof. Bingsen Zhang, Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, Liaoning, China. E-mail: bszhang@imr.ac.cn
How to cite this article: Wang Y, Niu Y, Pu Y, Li S, Liu Y, Zhang B. Revealing the dynamic formation mechanism of porous Mo2C:
an in-situ TEM study. Chem Synth 2023;3:42. https://dx.doi.org/10.20517/cs.2023.33
Received: 4 Jul 2023 First Decision: 28 Aug 2023 Revised: 11 Sep 2023 Accepted: 28 Sep 2023 Published: 10 Oct 2023
Academic Editors: Ying Wan, Jun Xu Copy Editor: Pei-Yun Wang Production Editor: Pei-Yun Wang
Abstract
In-situ transmission electron microscopy (TEM) enables direct observation of the micromorphology and
microstructure evolution of catalysts in the chemical atmosphere. Studying the structural evolution during the
formation of molybdenum carbide using in-situ TEM is helpful for the preparation of high-performance carbide
catalysts. Herein, the formation mechanism of porous Mo2C from MoO2 nanoparticles (NPs) was studied by in-situ
TEM. The formation of Mo2C was induced by the defects of MoO2, and the formed Mo2C facilitated the
carbonization of neighboring MoO2 NPs. The growth rate of Mo2C between MoO2 NPs was slower compared to
that within a single MoO2 NP. In addition, the formation and growth of pores in Mo2C were also studied; the pores
grew radially during the early stages from the nucleation sites and later grew branched and curved. As Mo2C
underwent competitive growth, the pores transitioned from straight to curved. Eventually, during prolonged
carbonization at high temperatures, Mo2C underwent sintering.
Keywords: Mo2C, porous structure, growth mechanism, in-situ TEM
Page 2 of 11 Wang et al. Chem Synth 2023;3:42 https://dx.doi.org/10.20517/cs.2023.33
INTRODUCTION
The transition metal carbides are widely used in industry due to their unique physical and chemical
properties[1-7]. For instance, molybdenum (Mo) carbides can serve as supports in solid catalysts, which
exhibit good performances in heterogeneous catalysis[8-10]. Moreover, Mo carbides also are the active phases,
owning similar catalytic properties with the precious metals because of the introduction of carbon
atoms[11-18]. They have been applied in hydrogenation[19-23], steam reforming of methanol[24], and water gas
shift reaction[25]. Among them, β-Mo2C demonstrated excellent activity and selectivity in CO2 reduction[26],
hydrogen production[27], and hydrodeoxygenation reactions[28]. Therefore, the research and development of
preparation technology plays an important role in the application of high-performance Mo carbide
catalysts. At present, the main preparation methods of Mo carbides include solid-solid and solid-gas
(temperature-programmed reduction) reaction methods[29]. The solid-gas approach, which employs the
reaction of the Mo oxides and the carbon-containing gases (e.g., CO, CH4, C2H6, and aromatic compounds),
was developed rapidly[30-36]. It is crucial to investigate the structural transformation process of Mo oxides
into Mo carbides during temperature-programmed reduction carbonization.
Along with the development of preparation technologies for Mo carbides, the carbonization mechanism of
Mo oxides to Mo carbides has also been studied intensively[37-39]. The carburization of MoO3 with
hydrocarbon and hydrogen was studied by the photoelectron spectroscopy[40]. The carbon deposition on the
sample surface decreased with increasing carbon content. It indicated the diffusion of surface carbon on the
sample into a bulk phase in the carburization process of Mo oxides. The MoO2 was the intermediate phase
determined by thermodynamic analysis in the process of MoO3 transformed into Mo2C[41]. Based on this, the
structural evolution processes of MoO2 in different carbon-containing gas atmospheres were investigated
systematically. For example, the reduction of MoO2 powders with CO to produce Mo2C was studied, and
the reduction mechanisms were significantly distinct at different temperatures[42]. The MoO2 followed a one-
step reaction translated to Mo2C at lower temperatures. However, the transition process was different at
high temperatures; the MoO2 was reduced to Mo first, and then the Mo was carburized to Mo2C. The
transformation of MoO2 to Mo2C in a hydrocarbon atmosphere was also focused. The MoOxCy as the
intermediate phases were observed during the carburization of MoO2 in CH4/H2 or C2H6/H2[43]. In addition,
the mechanism of MoO2 to Mo2C under methane pulse conditions was also studied[44]. The researchers
proposed the “plum-pudding” model in the solid-phase transformation from MoO2 to Mo2C. However, due
to the limitation of characterization techniques, there are no relevant reports on the study of direct
observation regarding Mo2C formation processes in the preparation by temperature-programmed reduction
methods. It seriously affects the perception and regulation of Mo2C preparation. Usually, the direct reaction
of Mo oxides with a carbon source at elevated temperatures generates β-Mo2C. The investigation of their
structural formation mechanism could provide guidance for the controllable preparation and also benefit
the synthesis of Mo2C with other phase structures.
In-situ transmission electron microscopy (TEM) has been used to directly visualize the structural evolution
of nano-catalysts[45-47]. In recent years, the deoxidation process of MoO3 was studied by environmental TEM,
accompanied by the nanoparticles (NPs) crushing and regrowth in H2 and thermal field, and the MoO3 has
a good directional deoxidation process[48]. Meanwhile, the different solid carbon sources were used, and the
controllable growth of Mo carbides was monitored by in-situ TEM[49]. The face centered cubic (FCC) MoC
with full occupation of interstitial sites by carbon sites was formed because the sucrose was decomposed
into high reactivity sp3carbon atoms. The three-stage mechanism during nucleation and growth of Mo2C
NPs was also revealed by in-situ TEM[50]. Furthermore, if an atmosphere is introduced into TEM, the
formation process of Mo carbides in the carbonation atmosphere can be directly observed. This helps to
understand the mechanism of synthesizing Mo carbides by the solid-gas reaction method.
Wang et al. Chem Synth 2023;3:42 https://dx.doi.org/10.20517/cs.2023.33 Page 3 of 11
Herein, to get a better understanding of the formation mechanism of Mo oxides to Mo carbides in a
temperature-programmed reduction method, we studied the microstructural evolutions of MoO2 NPs to
porous Mo2C in a mixed atmosphere of methane and hydrogen by using in-situ TEM. And the mechanism
of gas-solid reaction in the formation of Mo2C was analyzed in detail.
EXPERIMENTAL
Synthesis of MoO3 nanobelts
The typical solvothermal method applied to synthesize the MoO3 nanobelts has been reported previously[51].
In brief, the 4,839 mg (0.02 mol) sodium molybdate dihydrate was dissolved in 60 mL deionized water after
the 4 mL nitric acid was diluted by 16 mL deionized water and added to the stirred solution dropwise. Then,
the solution was transferred into a 100 mL autoclave, sealed, and maintained at 130 °C for 12 h in an oven.
Finally, the obtained deposition was filtered and washed with deionized water and then calcinated at 300 °C
in air for 2 h. The MoO2 was prepared by reducing MoO3 nanobelts in H2 or CH4/H2 atmosphere at 500 °C
for 2 h with a heating rate of 10 °C/min. Mo2C was synthesized under a CH4/H2 atmosphere at 700 °C for
2 h.
Samples characterization
TEM and high-angle annular dark field scanning TEM (HAADF-STEM) images were obtained using FEI
Tecnai G2 F20 operated at 200 kV. Scanning electron microscopy (SEM) images were obtained using
Regulus 8100 operated at 3 kV. X-ray diffraction (XRD) measurements were performed on a Rigaku D/max
2400 diffractometer (Cu Kα radiation, λ = 0.15418 nm) operating at 40 kV and 40 mA. X-ray photoelectron
spectroscopy (XPS) characterization was carried out with an ESCALAB 250 instrument with Al Kα X-rays
(1,489.6 eV, 150 W, 50.0 eV pass energy). N2 adsorption-desorption isotherms were measured on an ASAP
2020 micromeritics apparatus, and the specific surface areas of the samples were calculated following the
multi-point BET (Brunauer-Emmett-Teller) procedure. The pore-size distributions were determined from
the adsorption branch of the isotherms using the BJH (Barett-Joyner-Halenda) model.
CH4/H2 temperature programmed surface reaction (TPSR)
The 40 mg MoO3 nanobelts were loaded in a quartz tube reactor that was connected to a mass spectrometer.
Following, the samples were elevated to 700 °C with a heating rate of 5 °C/min under 20 vol.% CH4/H2 flow
(5 mL/min). The mass signals of H2 (Mz = 2) and H2O (Mz = 18) were monitored during the process.
In-situ TEM investigation
The commercial gas-heating holder (produced by DENSsolutions), a home-made gas controlling system,
and the Titan Themis G3 ETEM were adopted during the in-situ TEM experiments. The MoO3 sample was
encapsulated in a nanoreactor of a gas-heating holder. The nanoreactor has two amorphous SiNx windows
in order to observe the structural evolution of MoO3 catalysts. The sample was calcinated at 300 °C in
20 vol.% O2/He atmosphere for 30 min to remove any potential contamination before in-situ TEM
experiments. Then, the temperature was lowered to room temperature, and the gas was switched to a
20 vol.% CH4/H2 atmosphere. The pressure in the nanoreactor was 1 bar during the test. Following, the
temperature was sequentially elevated and kept at 500 °C, 600 °C, 700 °C, and 750 °C for in situ structural
characterization.
RESULTS AND DISCUSSION
Characterization of the Mo oxides (carbides)
The typical XRD patterns and TEM images of MoO3 nanobelts after calcination are shown in
Supplementary Figure 1. The XRD pattern [Supplementary Figure 1A] confirmed the MoO3 phase structure
(JCPDS 65-2421) for MoO3 nanobelts calcinated at 300 °C, and the sharp diffraction peaks verified the high
Page 4 of 11 Wang et al. Chem Synth 2023;3:42 https://dx.doi.org/10.20517/cs.2023.33
crystallinity. High-resolution TEM (HRTEM) with local fast Fourier transform (FFT) observation also
identified the crystallographic property of MoO3. The interplanar distances of 0.380 nm and 0.369 nm with
an intersection angle of 90° were assigned to the (101) and (010) planes [Supplementary Figure 1C]. The
sharp, bright diffraction spots are displayed in FFT [Supplementary Figure 1D], illustrating the high
crystallinity of MoO3 nanobelts, too. The temperature programmed surface reaction (TPSR) experiments
show that the content change curves of H2 and H2O when MoO3 nanobelts were reduced and carbonized
[Supplementary Figure 2]. The reduction peaks at 500 °C and 600 °C indicate MoO3 reduced to MoO2 and
MoO2 transformed to Mo2C, respectively. The second peak area of H2O was larger than the first peak of
H2O, suggesting that there was a lot of water generated during the transformation from MoO2 to Mo2C.
SEM images were used to show the morphology of MoO2 and Mo2C samples in Supplementary Figure 3.
The produced MoO2 by reducing MoO3 nanobelts in the CH4/H2 atmosphere at 500 °C exhibited a plate-like
shape. As the treatment temperature increased, the MoO2 was carbonized to Mo2C at 700 °C. The presence
of porous agglomerates Mo2C built up by MoO2 platelets was observed. The MoO2 and Mo2C phases were
confirmed by XRD [Supplementary Figure 3C and F] according to JCPDS 65-5787 and JCPDS 35-0787,
respectively.
Observation regarding the formation of porous Mo2C
In order to understand the formation mechanism of Mo2C, we observed the micromorphological and
microstructural evolution of MoO2 to Mo2C at the nanoscale using in-situ TEM. Figure 1 exhibits the
carbonization process of MoO2 to Mo2C under the 20 vol.% CH4/H2 atmosphere at ambient pressure.
Figure 1A shows that the samples were MoO2 NPs under the CH4/H2 atmosphere at 600 °C. The average
particle size of MoO2 NPs was 62.8 nm [Supplementary Figure 7]. When the temperature increased to
700 °C, the porous nanocrystals (pointed out with arrows) were observed, as shown in Figure 1B, suggesting
the formation of Mo2C at this temperature. Subsequently, the porous Mo2C grew radially [Figure 1C]. As
displayed in Figure 1D, most of MoO2 NPs had been transformed into porous Mo2C. Figure 1E shows the
corresponding selected area electron diffraction (SAED) patterns of MoO2 NPs [Figure 1A] and porous
Mo2C [Figure 1D] in the CH4/H2 atmosphere. The d-spacings of 3.42 Å, 2.43 Å, and 1.71 Å are close to the
d(011), d(020), and d(022) referring to monoclinic MoO2 (JCPDS 65-5787). Alternately, the d-spacings of 2.37 Å,
1.75 Å, and 1.30 Å are close to the d(002), d(102), and d(200) of hexagonal Mo2C (JCPDS 35-0787). The diffraction
spots transform into diffraction rings when MoO2 is transformed into Mo2C, indicating that the porous
Mo2C is polycrystalline. Figure 1F exhibits the intensity profiles from the integration of SAED patterns in
Figure 1E, and it also confirmed the monoclinic MoO2 and hexagonal Mo2C pure phase structures for the
samples in the CH4/H2 atmosphere at 600 and 700 °C, respectively. The unit cells [Figure 1G] and the
histogram of cell volume and volume per Mo [Figure 1H] of MoO2 and Mo2C indicate that the formation of
pores may be attributed to the volume reduction due to phase transformation, as there will be about 58.06%
volume reduction when the same amount of Mo in the form of MoO2 is converted to Mo2C.
The STEM images and energy-dispersive X-ray spectroscopy (EDX) elemental maps of MoO2 and Mo2C
samples were displayed in Supplementary Figure 4, exhibiting the well-distributed Mo/O in MoO2
[Supplementary Figure 4A-C] and Mo/C in Mo2C [Supplementary Figure 4D-F]. It indicated that the Mo
oxides were completely transformed into the Mo carbides. TEM images and the EEL spectra of MoO2
and Mo2C samples [Supplementary Figure 5] show the morphology and electron structure changes of the
samples before and after carbonization. It was observed that in plate-shaped MoO2, there were Mo M-
edges and O K-edges, while in porous Mo2C, there were Mo M-edges and C K-edges, confirming the
presence of pure phase structures of MoO2 and Mo2C. The XPS results were demonstrated and
compared in Supplementary Figure 6. The peaks at 229.2 eV and 228.0 eV were assigned to Mo4+ and
Mo2+, respectively. As the MoO2 transformed to Mo2C, the content of Mo2+ species increased while the
Mo4+ species highly decreased. The presence of Mo5+(Mo6+) species is ascribed to the surface oxidation
while expositing to air.
Wang et al. Chem Synth 2023;3:42 https://dx.doi.org/10.20517/cs.2023.33 Page 5 of 11
Figure 1. In-situ transformation from MoO2 to Mo2C. (A-D) In-situ TEM images of the Mo2C formation from MoO2 in 20 vol.% CH4/H2
atmosphere; (E) The corresponding SAED patterns of MoO2 (A) and Mo2C (D); (F) Intensity profiles from the integration of diffraction
rings of SAED patterns in E (a.u., arbitrary units); (G) Unit cells of MoO2 and Mo2C; (H) The histogram of cell volume and volume per
Mo of MoO2 and Mo2C. Mo: Molybdenum; SAED: selected area electron diffraction; TEM: transmission electron microscopy.
Unraveling the nucleation and growth of porous Mo2C
Figure 2 displays the Mo2C nucleation process of MoO2 NPs under the drive of CH4 and H2 at 700 °C.
Previous studies have illustrated that the methane was cracked into carbon with high reactivity at high
temperatures, and the carbon species adsorbed on the surface of metal oxides[52,53]. In this experiment, the
MoO2 and highly reactive carbon further reacted and formed the Mo carbides. It is noteworthy that the
defects in MoO2 NPs were more likely to be carbonized preferentially. The preferential nucleation of Mo2C
was located at the defects of MoO2 NPs. Figure 2F showed many small nanocrystals gradually appearing and
the generation of pores at the defects of MoO2 NPs, suggesting that the Mo2C sites were formed here.
Following, Mo2C grew radially from the nucleation site in all directions at MoO2 NPs [Figure 2H-J]. It
proved that the defects in MoO2 NPs induced the nucleation of Mo2C. In addition, the carbonization
process of two adjacent MoO2 NPs was observed at 700 °C in the CH4/H2 atmosphere [Supplementary
Figures 8 and 9]. Under the co-drive of CH4 and H2, the defects in MoO2 NPs were carbonized first, and the
NP transformed into porous Mo2C. Subsequently, the carbonized MoO2 NP induced the carbonization of
adjacent MoO2 NPs at the interface between two NPs. The carbonization reaction crossed the interface
between two NPs and continued in the next MoO2 NP. Lastly, the two MoO2 NPs were turned to porous
Mo2C, and the grain boundary of the two NPs disappeared. It further reveals that the carbonization of
MoO2 NPs was also promoted by the adjacent already carbonized porous Mo2C as the nucleating agents.
Generally, the MoO2 NPs obtained by the reduction of MoO3 nanobelts were usually stacked[54]. Herein, the
growth rate of Mo2C within and between MoO2 NPs was studied [Figure 3]. There are approximately nine
MoO2 NPs stacked together in the observed two-dimensional (2D) region in Figure 3A. Figure 3K shows
the growth rate of 2D area of porous Mo2C in Supplementary Figure 10. The carbonization rate of
MoO2 NPs was represented by the growth rate of 2D area of porous Mo2C. When Mo2C grows across the
interface between two MoO2 NPs, it is considered to be carbonized between MoO2 NPs. Alternately,
when Mo2C only grows inside MoO2 NPs, it is carbonized within MoO2 NPs. Judging from TEM
images at 5 s, 10 s, and 15 s [Figure 3B-D], the carbonization reaction process between MoO2 NPs was
hindered and slowed down. This is mainly due to the presence of a high energy barrier at the grain
boundary[55,56]. Compared with Figure 3D (15 s) and E (20 s), the Mo2C growth rate within MoO2 NPs
Page 6 of 11 Wang et al. Chem Synth 2023;3:42 https://dx.doi.org/10.20517/cs.2023.33
Figure 2. Defect -induced nucleation of Mo2C. (A-E) In-situ TEM images of the nucleation process of Mo2C under 20 vol.% CH4/H2
atmosphere at 700 °C; (F-J) Enlarged TEM images acquired from the dashed boxes in (A) to (E). TEM: Transmission electron
microscopy.
Figure 3. Growth rate of Mo2C within and between MoO2 NPs. (A-J) In-situ TEM images of structural evolution from stacked MoO2 NPs
to Mo2C under 20 vol.% CH4/H2 atmosphere at 700 °C; (K) The carbonization rates within and between MoO2 NPs. The boxes
represent the growth rate of two-dimensional area of porous Mo2C. Blue numbers represent the number of interfaces with Mo2C
growth crossing between MoO2 NPs, and the capital letters represent the corresponding TEM images in (B-J). NPs: Nanoparticles;
TEM: transmission electron microscopy.
increased after the carbonization reaction of MoO2 crossed the interfaces between MoO2 NPs. Especially
from the 30 s [Figure 3F] to 35 s [Figure 3G], the growth of Mo2C was quick, and the growth rate of 2D
area of porous Mo2C is over 3,000 nm2/s. It was because of only the growth of Mo2C within MoO2 NPs
during this period. After 35 s [Figure 3H-J], the growth of Mo2C slowed down since Mo2C growth
crossed the interfaces between MoO2 NPs and the reduction of Mo sources in a late stage of MoO2
carbonization. Supplementary Figure 11 also exhibits that the growth rate of Mo2C within MoO2 NP was
slower than that between MoO2 NPs. The above results indicated that the presence of an interface
between MoO2 NPs could only slow down the carbonization process but not prevent it. After the
carbonization reaction within and between MoO2 NPs, the MoO2 NPs were transformed into porous Mo2C,
and the interfaces between the original MoO2 NPs disappeared.
Wang et al. Chem Synth 2023;3:42 https://dx.doi.org/10.20517/cs.2023.33 Page 7 of 11
Figure 4. Growth types during the formation of Mo2C. In-situ TEM images of radial (A-E), competition (B-J), and dendrite (K-O) growth
of Mo2C under 20 vol.% CH4/H2 atmosphere at 700 °C. TEM: Transmission electron microscopy.
Revealing the pore formation and evolution of porous Mo2C
Subsequently, the formation of pores in the growth of Mo2C was observed in Figure 4. The three types of
porous Mo2C growth were summarized. At the early stage, the pores showed radial growth from the center
to the periphery [Figure 4A-E]. The structural transition from MoO2 to Mo2C was accompanied by the
formation of straighter pores and faster pore growth rates. The competitive growth of pores was observed
when two Mo2C nucleation sites met during the carbonization process. As shown in Figure 4F-J, the
straighter pores became distorted, and the growth rate of pores was slowed down during this process. In
addition, the tip splitting and side branches of pores were observed in the late stage of growth. The growth
of porous Mo2C slowed down, and the pores curved due to the reduction of Mo sources [Figure 4K-O]. The
above observations revealed that the growth and evolution of pores were affected by the amount of Mo
sources. The pores of the Mo2C radial grew and formed straighter pores when the Mo sources were
sufficient. On the contrary, the pores of Mo2C became curved and branched when the Mo sources were
reduced.
Analyzing the microstructure of porous Mo2C
Lastly, the micromorphology and structure of porous Mo2C obtained by MoO2 carbonization were
analyzed, as shown in Figure 5. Figure 5B demonstrates the porous Mo2C with different orientations of
nanocrystals; the (101) facets with a lattice spacing of 2.29 Å were observed. The measured planes with
lattice spacings of 2.61 Å, 2.29 Å, and 2.29 Å and angles of 64° and 52° in Figure 5C correspond to the crystal
parameters of (010) 2.61 Å, (101) 2.29 Å and (1-11) 2.29 Å in hexagonal Mo2C structures (JCPDS 35-0787).
FFT pattern [Figure 5C] shows the hexagonal Mo2C structure from the [-101] zone axis [Figure 5D]. The
micromorphology and pore size of Mo2C nanocrystal were computed in Supplementary Figure 12. The
mean pore width and length are 1.4 nm and 9.7 nm, respectively. In addition, the N2 physisorption tests
[Supplementary Figure 13] were performed on MoO2 and Mo2C obtained after treatment at 600 and 700 °C,
respectively. The low surface area of 1.11 m2·g-1 MoO2 NPs translated to high surface area of 5.00 m2·g-1
porous Mo2C. The pore-size distribution of MoO2 showed a weak peak at 41.27 nm, whereas a strong peak
at 2.21 nm was observed over porous Mo2C, indicating that the pore formation in Mo2C samples. However,
Page 8 of 11 Wang et al. Chem Synth 2023;3:42 https://dx.doi.org/10.20517/cs.2023.33
Figure 5. Characterization of the porous Mo2C. In-situ TEM (A) and HRTEM (B and C) images of Mo2C under 20 vol.% CH4/H2
atmosphere at 700 °C; (D) The corresponding FFT patterns of (C). FFT: Fast Fourier transform; HRTEM: high-resolution TEM; TEM:
transmission electron microscopy.
the carbonization temperature of Mo2C prepared by a temperature-programmed reduction method should
not be too high. For example, Mo2C samples undergo obvious sintering and disappearance of pores when
the carbonization temperatures were increased to 750 °C from 700 °C, as shown in Supplementary Figure 14
.
CONCLUSIONS
The structure evolution process of MoO2 to porous Mo2C in a carbonizing atmosphere was systemically
investigated by using in-situ TEM. The formation of porous structures was observed during the
carbonization process, which could be attributed to the volume decrease when an equivalent of Mo in the
form of MoO2 is converted to Mo2C. The defects in MoO2 NPs facilitated the nucleation of Mo2C under
CH4/H2 atmosphere at 700 °C. The porous Mo2C induced the carbonization of neighboring MoO2 NPs, and
the interfaces between MoO2 NPs slowed down the carbonization process. The carbonization reaction
progressed from one MoO2 NP to another through the interface. In the early growth stage of porous Mo2C,
the pores exhibited radial growth from the nucleation sites. However, in the late stage, the tip splitting and
side branch formation of pores were observed. Moreover, the pores transitioned from straight to curved
when the porous Mo2C at different sites met during growth. Compared to MoO2, the porous Mo2C obtained
through temperature-programmed reduction methods has a higher specific surface area. This work sheds
light on the formation process of porous Mo2C in a CH4/H2 atmosphere and could provide guidance for the
synthesis of porous carbides.
DECLARATIONS
Authors’ contributions
Design of the study and writing of the manuscript for the whole work: Wang Y, Niu Y, Zhang B
Performed the TEM characterization: Wang Y, Niu Y, Li S, Liu Y
Performed the analysis of data: Wang Y, Niu Y, Pu Y
Performed data acquisition and provided administrative, technical, and material support: Wang Y, Niu Y,
Zhang B
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (Nos. 22072164, 22002173,
52161145403), the China Postdoctoral Science Foundation (2020M680999), the Natural Science Foundation
of Liaoning Province (2022-MS-004), and the foundations of Shenyang National Laboratory for Materials
Science.
Wang et al. Chem Synth 2023;3:42 https://dx.doi.org/10.20517/cs.2023.33 Page 9 of 11
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2023.
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