- Email: [email protected]
Preparation and process investigation of molybdenum carbide and their N-doped analogue by calcination
Journal Pre-proof Preparation and process investigation of molybdenum carbide and their N-doped analogue by calcination Yaqiu Tao, Shuaishuai Zhu, Zhigang Pan, Simin Qiu, Xiaodong Shen PII:
S0022-4596(19)30466-9
DOI:
https://doi.org/10.1016/j.jssc.2019.120961
Reference:
YJSSC 120961
To appear in:
Journal of Solid State Chemistry
Received Date: 23 November 2018 Revised Date:
12 September 2019
Accepted Date: 16 September 2019
Please cite this article as: Y. Tao, S. Zhu, Z. Pan, S. Qiu, X. Shen, Preparation and process investigation of molybdenum carbide and their N-doped analogue by calcination, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.120961. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Preparation and Process Investigation of Molybdenum Carbide and their N-doped Analogue by Calcination Yaqiu Taoa,b, Shuaishuai Zhua, Zhigang Pana,b *, Simin Qiua, Xiaodong Shena a
College of Materials Science and Engineering, Nanjing Tech University, 30 South Puzhu Road,
Nanjing, Jiangsu Province 21009, PR China b
State Key laboratory of Materials-Oriented Chemical Engineering, 30 South Puzhu Road, Nanjing,
Jiangsu Province 21009, PR China Corresponding author's email: [email protected] Abstract The crystal structure of Mo2C can be derived from close packed Mo atoms with partial occupation of C atoms in the octahedral interstices. Mo2C and MoC were successfully synthesized by calcinaiton of the lab-made
precursor
containing
1,8-diaminonaphthalene
and
ammonium
molybdate
at
carbon/molybdenum ratio of 2.0, 2.25, 2.7 and 4.5, respectively. High temperature XRD was used in the formation process study of Mo2C, which presents MoO2 as an important intermediate in formation of Mo2C. A reduction of the carbon/molybdenum ratio to 1.8 in the precursor caused shifts of XRD reflections in Mo2C-0.4. Rietveld refinement of the XRD reflections of Mo2C-0.4 reveals a partial nitrogen substitution of carbon atoms in Mo2C, which is confirmed by the theoretical calculation. The theoretical carbon and nitrogen contents of Mo2N0.6C0.4 agree with the experimental carbon and nitrogen content results of Mo2C-0.4 from element analysis. This confirms the substitution of carbon by nitrogen in the Mo2C lattice. Introduction Transition metal carbides are applicable in many cases due to their outstanding physicochemical properties and received more attention in the last decade than ever. Molybdenum carbides are found active in many catalytic applications. MoC presents promotion effects in Au and Pt catalyzed H2 production reactions1, 2. Mo2C nanowires and Mo2C modified carbon nano tube materials are synthesized by calcination in an inert atmosphere and used in methanol decomposition3-5. Mo2C/carbon composite materials are used in lithium storage6 and H2 generation from formic acid7. Mo2C catalysts also present high reactivity in both the hydrogen evolution reaction8, 9 and acetone hydrodeoxygenatin10. The oxidation of Mo2C to MoO2 is recognized as the main cause of the deactivation of this catalyst in methane reforming reactions11.
Besides the traditional temperature programmed reduction12-18 synthesis of Mo2C, a new thermal decomposition method of organic-inorganic hybrids is also studied in the past decade. Organic compounds including hexamethylenetetramine19-21, aniline22，glucose23 and biomass24 are used as carbon sources in the annealing preparation of Mo2C materials. A relatively close contact between Mo and ligands are presented in the molybdenum formamidinate complexes
25
, which provides a short
molybdenum carbon distance in the precursor for calcination. Both cubic (fcc) MoC and orthorhombic (hcp) Mo2C26 phase molybdenum carbides are found in the products of calcination. Carbon/metal ratio in the precursor plays an important role in modifying the crystal phase of the annealing products20, 22, 27. A 2D Mo2C material is prepared by chemical vapor deposition28, the STEM study of this material presents an orthorhombic structure. The MoC phase product is formed by calcination with a precursor at a higher carbon/metal ratio29. Heteratom including N and P doped Mo2C cause interests to more by their unique catalysis properties30. A nitrogen doped Mo2C is prepared by31 nitrogen rich ligands such as cetyltrimethyl ammonium bromide31,32. At temperatures higher than 1960℃, Mo2C possesses a high symmetry space group P63/mmc (No.194). In a P63/mmc unit cell, the two Mo atoms are located at the Wyckoff position 2c. The hexagonal structure is transformed to P 31m (No. 162) at 1350-1690℃. At temperatures lower than 1350℃, the trigoral structure transfers to Pbcn (No. 60). In this orthorhombic structure, Mo atoms are in general positions while the four carbon atoms are located in Wyckoff 4c with the coordinates (0,y,1/2), (1/2,-y+1/2,3/4), (0,-y,3/4) and (1/2,y+1/2,1/4) (where x = 0.375). Our previous study showed that Mo2C on active carbon present high selectivity and activity in H2 generation from formic acid decomposition7. The formation of appropriate molybdenum carbide phase is crucial in the catalytic activity study. In this work, a serial of molybdenum carbide sample was prepared by modifying the carbon/molybdenum ratio in the precursor for Mo2C or MoC phase synthesis. Mo2C dominate the calcinaiton products at the carbon/molybdenum ratio of 2.0, 2.25 and 2.7. A high temperature XRD study of this 1,8-Diaminonaphthalene contained molybdenum precursor reveals MoO221,
33
as an important intermediate in the formation of Mo2C. MoC is prepared by
calcination of the precursor containing excess carbon source. A further reduction of the carbon/molybdenum ratio to 1.8 in the precursor caused shifts of XRD reflections in Mo2C-0.4. Rietveld refinement of these XRD reflections of Mo2C-0.4 reveals a partial substitution of the carbon atoms by nitrogen in Mo2C. This is confirmed by theoretical calculation and elemental analysis of
Mo2C-0.4. Experiment Preparation of Molybdenum carbides and analogue The molybdenum carbides and analogue were synthesized via a temperature programmed calcination of lab-prepared precursor of molybdenum under N2. Ammonium molybdate was used as the molybdenum source and 1,8-diaminonaphhalene was used as the carbon source of these carbides. An aqueous solution of ammonium molybdate was added dropwise to the ethanol solution of 1,8-aminonaphhalene at pH=1. The remainder obtained by drying the mixed solution was calcinated under N2 at 750
for 5h to prepare the desired carbide.
Characterization Methods All the prepared carbides were analyzed by N2 adsorption/desorption using a ASPS2020 accelerated surface area porosimetry from Micromeritics. The specific areas were determined by applying the Brunauer-Emmet-Teller (BET) method in the relative pressure p/p0 range between 0.05 and 0.3. Power X-ray diffraction was carried out on a Smartlab 9Kw diffractionmetre in reflection geometry (Ni-Filtered, λ = 1.5418 Å). A heating rate of 20℃/min were used in the high temperature XRD spectra study under N2. The XRD reflection of the calcination product were detected after heated to a set temperature and remained at the same temperature till the end of the XRD signal collection. The morphologies study was performed on a JEOL LSM-6510 scanning electron microscope (SEM) coupled with a EDX detector from Thermo Scientific. In the Rietveld refinement, the orthor-Mo2C crystal structure was taken as the initial model. Only the phase scale factor and six coefficients of a shifted-Chebyshev polynomial to approximate the background were refined. Lattice parameters, peak shape parameters, grain-size peak broadening were then allowed to refine. No atomic parameter was refined. Attempts to refine the isotropic displacement parameters lead to non-meaningful values and therefore were not included in the final Rietveld refinement. The first principle calculations of the structural parameters of N doped Mo2C have been performed using the Cambridge Serial Total Energy Package (CASTEP) employing the periodic density functional theory. The Generalized Gradient Approximation (GGA) using the PBE (Perdew–Burke–Ernzerhof) functional was chosen to account for Exchange–correlation effects. The ionic cores were described using ultra-soft pseudopotentials, with a plane wave cutoff of 360 eV. For this calculation, a Monkhorst-Pack scheme has been used to sample the Brillouin zone; a 3 × 3 × 3 k-point mesh size was employed to properly describe bulk Mo2C. Tolerances for energy convergence and forces were set at
5x10-4 and 0.01 eV/Å, respectively34. Result and discussion XRD Characterization and SEM imaging As described in the experimental section, the carbon/molybdenum ratio of the precursor were carefully controlled, which results in different crystal phase. The carbon/molybdenum ratio of 1.8:1, 2.0:1, 2.25:1, 2.7:1 and 4.5:1 are used and the products obtained are labeled as Mo2C-0.4, Mo2C-0.45, Mo2C-0.5, Mo2C-0.6 and MoC-1.0. The XRD patterns of the products obtained from different carbon/molybdenum ratio are presented in Figure 1a. All the XRD patterns of the products obtained from calcination are similar except the high carbon/molybdenum precursor produced a different molybdenum carbide phase. Compared to the database, the XRD pattern of MoC-1.0 presents 2θ signals of 36.6º, 42.38º, 61.64º, 73.45º and 77.60º, which is consistent with MoC crystal phase, shown in Figure 1b. Nano sized MoC crystalline was formed from calcination. With a lower carbon/molybdenum ratio at 2.0:1, 2.25:1and 2.7:1, the Mo2C crystalline phase was observed from the XRD pattern, Figure 1c. The XRD signals are slightly broadened compared to the reference (PDF79-744 ). Though the XRD pattern of Mo2C-0.4 is similar to that of Mo2C-0.5, a close inspect of the XRD data reveals shifts of several signals at 1.8:1 carbon/molybdenum ratio, shown in Figure2 d. The decrease of carbon source caused the changes in d-spacing, as indicated by the shifts of the XRD reflections. The structure refinement of this compounds is discussed in detail in the follow text. High temperature XRD A high temperature XRD under N2 is used in the forming process study of molybdenum carbide from the precursor. A platinum plates was used as the heating plates and XRD testing substrate. When heated to 290℃, a hexagonal MoO3 phase was formed as presented in Figure 2a. The pyrolsis product remained as hexagonal phase MoO3 until the precursor was heated to 450℃. At 450℃, a phase transfer of hexagonal MoO3 to orthorhombic MoO3 was detected and some molybdenum oxide with a formula of MoO2-3 intermediate phase is also formed which indicated a losing of oxygen atom in the molybdenum oxide lattice and a reduction of Mo(III) to Mo(II). A further increase of the intermediate MoO2-3 phase is detected in the XRD pattern at 550℃ with a reduction of the orthorhombic MoO3 phase. A further treatment to 650℃ and 700℃, the precursor is converted to a relatively pure MoO2 phase,
which is a main intermediate phase to form Mo2C. The characteristic XRD signal of Mo2C at 2θ=39.5º was first observed at 750℃, and the intensity of this signal kept increasing till the end of the high temperature XRD study to 850℃, shown in Figure 2b. The high temperature XRD study reveals MoO2 as an important intermediate in the formation of Mo2C phase. Mo(III)
in the precursor was first
reduced to Mo(II) before converted to Mo2C. Organic carbon source, eg 1,8-Diaminonaphthalene, does not only contribute as the carbon source in these carbides but also as a reduction agent in converting Mo(III) to Mo(II) and Mo2C. Structure refinement of Mo2C-0.4 and theoretical calculation The symmetry reduction from P63/mmc to P 31m involves two steps. At first, the hexagonal symmetry is lost leading to a trigonal subgroup
3 1. The Wyckoff position 2a of the carbon atoms in P63/mmc
splits into two independent Wyckoff positions 1a and 1b in 3 1. In the second step, the subgroup
P 31m is obtained with a triple-sized unit cell. The Wyckoff position 1a in 3 1 splits into the Wyckoff positions 1a and 2c in P 31m while the Wyckoff position 1b splits into the Wyckoff positions 1b and 2d. As shown in Fig.3a, when the hex-Mo2C structure is transformed to the trig-Mo2C structure, the lattice parameters changes slightly and the fractional coordinates of all atoms remain the same while the occupancies for carbon atoms are varied. The symmetry reduction from P63/mmc to Pbcn involves three steps, In the first step, the hexagonal symmetry is lost resulting in an orthorhombic subgroup with a double sized C-centered cell. The centering is then removed in the second step and half of the translations is lost. In the third step, the a-axis is doubled and the orthorhombic subgroup Pbcn is obtained with interchange of axes. As a consequence, the corresponding Wyckoff position 2a of the carbon atoms in P63/mmc splits into two independent Wyckoff positions 4c in Pbcn, one of which is occupied by carbon atoms and the others are vacant, as shown in Figure.3b. In the synthesis of Mo2C and analogues, the metal/carbon ratio of the precursor were carefully controlled to obtain the desired products. As described previously the carbon/molybdenum ratio were managed from 1.8:1 to 4.5:1, which resulted in different products though the calcination were carried out under the same condition. A Mo2C phase material was successfully synthesized at a carbon/molybdenum ratio at 2.0:1, 2.25:1 and 2.7:1 as presented in the XRD spectra in Figure 1c. The Rietveld refinement for Mo2C-0.45 using generalized structure analysis system (GSAS) presents an orthorhombic unit cell where a=4.74Å, b=6.02Å and c=5.21Å, with a unit cell volume of 148.67 Å3, as
listed in Table 1. A refinement of the XRD data of Mo2C-0.4 , reveals a unit cell where a=4.78Å, b=5.96Å and c=5.15Å, with a unit cell volume of 146.72 Å3. As shown in Figure 1d, besides the unmoved signal at 2θ=52.1º, the unit cell distortion of Mo2C-0.4 cause a XRD signal shifts to lower angel at 2θ=37.8 º and 69.5º and higher angel for signals at 2θ=34.3º, 39.8º, 61.5º, 74.5º and 75.6º. In the geometric optimization by first principle calculation, the orthorhombic cell of Mo2C with a space group of Pbcn was used as the model. Nitrogen atom was used to substitute the carbon atom in the octahedron structure formed by molybdenum and carbon in the lattice. A set of model with the formula of Mo2C1~xNx (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1) were constructed and used in the structure refinements. As shown in Table 2, with the increase amount of N in the structure, the a-axis increases while both the b- and c-axis decreases with the volume of the unit cell. This results shows a consistency with that of the Rietveld refinements listed in Table 1. Take all these parameters into account, Mo2C-0.4 is consistent with the structure in which 60-70% carbon atoms substituted by N. The theoretical carbon and nitrogen contents of Mo2N0.6C0.4 are 2.3% and 4.0%, respectively. This result is agreed with the carbon and nitrogen contents results obtained from element analysis of Mo2C-0.4, shown in supporting information Table 1. Conclusion Mo2C and MoC were successfully synthesized by calcinaiton from the lab-made precursor of 1,8-diaminonaphthalene and ammonium molybdate at a carbon/molybdenum ratio of 2.0, 2.25 and 2.7 and 4.5, respectively. High temperature XRD was used in the formation process study of Mo2C; MoO2 is an important intermediate in formation of Mo2C. A reduction of the carbon/molybdenum ratio to 1.8 in the precursor caused shifts of XRD signals in Mo2C-0.4. Rietveld refinement of these XRD reflections of Mo2C-0.4 reveals the partial nitrogen substitution of carbon atoms in Mo2C, which is confirmed by theoretical calculation. The theoretical carbon and nitrogen contents of Mo2N0.6C0.4 is agreed with the carbon and nitrogen contents obtained from element analysis of Mo2C-0.4. This confirms the substitution of carbon by nitrogen in the Mo2C lattice. Acknowledgements We sincerely appreciate the financial support of grant from the Natural Science Funds for Young Scholar of China (Grant No.21107049) and the priority academic program development of Jiangsu Higher Education Institution (PAPD).
Reference 1.
Low-temperature hydrogen production from water and methanol using Pt/α-MoC catalystsL. Lin, W. Zhou, R. Gao, S. Yao, X. Zhang, W. Xu, S. Zheng, Z. Jiang, Q. Yu, Y.-W. Li, C. Shi, X.-D. Wen and D. Ma, Nature, 2017, 544, 80-83.
2.
Atomic-layered Au clusters on α-MoC as catalysts for the low-temperature water-gas shift reactionS. Yao, X. Zhang, W. Zhou, R. Gao, W. Xu, Y. Ye, L. Lin, X. Wen, P. Liu, B. Chen, E. Crumlin, J. Guo, Z. Zuo, W. Li, J. Xie, L. Lu, C. J. Kiely, L. Gu, C. Shi, J. A. Rodriguez and D. Ma, Science 2017, 357, 389-393.
3.
Synthesis of Nanoporous Molybdenum Carbide Nanowires Based on Organic-Inorganic Hybrid Nanocomposites with Sub-Nanometer Periodic StructuresQ. Gao, C. Zhang, S. Xie, W. Hua, Y. Zhang, N. Ren, H. Xu and Y. Tang, Chemistry of Materials, 2009, 21, 5560-5562.
4.
Preparation of supported Mo2C-based catalysts from organic–inorganic hybrid precursor for hydrogen production from methanol decomposition Q. Gao, C. Zhang, S. Wang, W. Shen, Y. Zhang, H. Xu and Y. Tang, Chem Commun (Camb), 2010, 46, 6494-6496.
5.
A nanoporous molybdenum carbide nanowire as an electrocatalyst for hydrogen evolution reaction L. Liao, S. Wang, J. Xiao, X. Bian, Y. Zhang, M. D. Scanlon, X. Hu, Y. Tang, B. Liu and H. H. Girault, Energy Environ. Sci., 2014, 7, 387-392.
6.
Synthesis, characterization and lithium-storage performance of MoO2/carbon hybrid nanowires Q. Gao, L. Yang, X. Lu, J. Mao, Y. Zhang, Y. Wu and Y. Tang, J. Mater. Chem., 2010, 20, 2807-2812.
7.
Large volume in situ H2 production on fixed bed reactor by concentrated formic acid aqueous solution Y. Tao, L. Tao, Z. Pan, S. Qiu and X. Shen, Fuel, 2018, 217, 106-110.
8.
3D Hierarchical Porous Mo 2 C for Effi cient Hydrogen EvolutionH. Ang, H. Wang, B. Li, Y. Zong, X. Wang and Q. Yan, Small, 2016, 12, 2859-2865.
9.
Mo2C Nanoparticles Dispersed on Hierarchical Carbon Microflowers for Efficient Electrocatalytic Hydrogen Evolution Y. Huang, Q. Gong, X. Song, K. Feng, K. Nie, F. Zhao, Y. Wang, M. Zeng, J. Zhong and Y. Li, ACS nano, 2016, 10, 11337-11343.
10.
Acetone Hydrodeoxygenation over Bifunctional Metallic−Acidic Molybdenum Carbide CatalystsM. M. Sullivan and A. Bhan, ACS Catalysis, 2016, 6, 1145-1152.
11.
Insights into the deactivation mechanism of metal carbide catalysts for dry reforming of methane via comparison of nickel-modified molybdenum and tungsten carbidesZ. Yao, J. Jiang, Y. Zhao, F. Luan, J. Zhu, Y. Shi, H. Gao and H. Wang, RSC Adv., 2016, 6, 19944-19951.
12.
S. Tuomi, R. Guil-Lopez and T. Kallio, Journal of Catalysis, 2016, 334, 102-109.
13.
Acid site densities and reactivity of oxygen-modified transition metal carbide catalystsM. M. Sullivan and A. Bhan, Journal of Catalysis, 2016, 344, 53-58.
14.
Low temperature CO2 hydrogenation to alcohols and hydrocarbons over Mo2C supported
15.
Cu-Mo2C/MCM-41: An Efficient Catalyst for the Selective Synthesis of Methanol from CO2 X.
metal catalystsY. Chen, S. Choi and L. T. Thompson, Journal of Catalysis, 2016, 343, 147-156. Liu, Y. Song, W. Geng, H. Li, L. Xiao and W. Wu, Catalysts, 2016, 6, 75. 16.
Reaction intermediate species during the steam reforming of methanol over metal modified molybdenum carbide catalystsJ. Cao, Y. Ma, G. Guan, X. Hao, X. Ma, Z. Wang, K. Kusakabe and A. Abudula, Applied Catalysis B: Environmental, 2016, 189, 12-18.
17.
R. Li, A. Shahbazi, L. Wang, B. Zhang, A. M. Hung and D. C. Dayton, Applied Catalysis A:
General, 2016, 528, 123-130. 18.
Mo2C catalyzed vapor phase hydrodeoxygenation of lignin-derived phenolic compound mixtures to aromatics under ambient pressureC.-J. Chen, W.-S. Lee and A. Bhan, Applied Catalysis A: General, 2016, 510, 42-48.
19.
One-step synthesis of bulk Mo and Ni–Mo carbides for methanation X. Huo, Z. Wang, J. Huang, R. Zhang and Y. Fang, RSC Adv., 2016, 6, 24353-24360.
20.
Bulk Mo and Co–Mo carbides as catalysts for methanation X. Huo, Z. Wang, J. Huang, R.
21.
The efficient synthesis of a molybdenum carbide catalyst via H2-thermal treatment of a
Zhang and Y. Fang, Catalysis Communications, 2016, 79, 39-44. Mo(VI)–hexamethylenetetramine complex Z.-Q. Wang, Z.-B. Zhang and M.-H. Zhang, Dalton Trans., 2011, 40, 1098-1104. 22.
Controllable synthesis of α-MoC1-x and β-Mo2C nanowires for highly selective CO2 reduction to CO J. Gao, Y. Wu, C. Jia, Z. Zhong, F. Gao, Y. Yang and B. Liu, Catalysis Communications, 2016, 84, 147-150.
23.
Ni/b-Mo2C as noble-metal-free anodic electrocatalyst of microbial fuel cell based on Klebsiella pneumoniae L. Z. Zeng, S. F. Zhao, Y. Q. Wang, H. Li and W. S. Li, Int. J. Hydrogen Energy, 2012, 37, 4590-4596.
24.
Selective hydrogenation of lignin to produce chemical commodities by using a biochar supported Ni–Mo2C catalyst obtained from biomassY.-Y. Wang, L.-L. Ling and H. Jiang, Green Chem., 2016, 18, 4032-4041.
25.
Dimolybdenum complexes with mixed formamidinate ligands Y.-Y. Wu, Y.-C. Kao, J.-D. Chen and C.-H. Hung, Inorganica Chimica Acta, 2004, 357, 1002-1010.
26.
Performances of mixed alcohols synthesis over potassium promoted molybdenum carbides M. Xiang, D. Li, W. Li, B. Zhong and Y. Sun, Fuel, 2006, 85, 2662-2665.
27.
An expeditious synthesis of early transition metal carbide nanoparticles on graphitic carbonsD. Ressnig, S. Moldovan, O. Ersen, P. Beaunier, D. Portehault, C. Sanchez and S. Carenco, Chemical communications, 2016, 52, 9546-9549.
28.
Unique Domain Structure of Two-Dimensional α‑Mo2C Superconducting CrystalsZ. Liu, C. Xu, N. Kang, L. Wang, Y. Jiang, J. Du, Y. Liu, X. L. Ma, H. M. Cheng and W. Ren, Nano letters, 2016, 16, 4243-4250.
29.
Synthesis of a-MoC1-x Nanoparticles with aSurface-Modified SBA-15 Hard Template: Determination of Structure–FunctionRelationships in Acetic Acid Deoxygenation F. G. Baddour, C. P. Nash, J. A. Schaidle and D. A. Ruddy, Angewandte Chemie, 2016, 55, 9026-9029.
30.
Pomegranate-like N,P-Doped Mo2C@C Nanospheres as Highly Active Electrocatalysts for Alkaline Hydrogen Evolution Y. Y. Chen, Y. Zhang, W. J. Jiang, X. Zhang, Z. Dai, L. J. Wan and J. S. Hu, ACS nano, 2016, 10, 8851-8860.
31.
Ultra-efficient electrocatalytic hydrogen evolution at one-step carbonization generated molybdenum carbide nanosheets/N-doped carbon C. Du, H. Huang, Y. Wu, S. Wu and W. Song, Nanoscale, 2016, 8, 16251-16258.
32.
Mo2C quantum dot embedded chitosan-derived nitrogen-doped carbon for efficient hydrogen evolution in a broad pH range Z. Pu, M. Wang, Z. Kou, I. S. Amiinu and S. Mu, Chemical communications, 2016, 52, 12753-12756.
33.
Hydrogen production by steam reforming of biomass tar over biomass char supported
molybdenum carbide catalyst M. Kaewpanha, G. Guan, Y. Ma, X. Hao, Z. Zhang, P. Reubroychareon, K. Kusakabe and A. Abudula, Int. J. Hydrogen Energy, 2015, 40, 7974-7982. 34.
Materials Studio 6.1; Accelrys, Inc., 2012.
Table 1 GSAS refinement of the Mo2C-0.4 and Mo2C-0.45 Lattice parameter
Mo2C-0.45
Mo2C-0.4
a/ Å
4.74
4.78
b/ Å
6.02
5.96
c/ Å
5.21
5.15
V/ Å3
148.67
146.72
Table 2 Theoretical calculation of N doped Mo2C structure Lattice parameter
Mo2C
Mo2N0.1C0.9 Mo2N0.2C0.8 Mo2N0.3C0.7 Mo2N0.4C0.6 Mo2N0.5C0.5 Mo2N0.6C0.4 Mo2N0.7C0.3 Mo2N0.8C0.2 Mo2N0.9C0.1
Mo2N
a/ Å
4.74
4.74
4.74
4.74
4.75
4.76
4.77
4.78
4.80
4.83
4.87
b/ Å
6.04
6.04
6.04
6.03
6.03
6.02
6.02
6.01
6.00
5.98
5.97
c/ Å
5.22
5.20
5.19
5.17
5.16
5.14
5.13
5.12
5.10
5.08
5.06
V/ Å3
149.36
148.87
148.43
148.03
147.69
147.40
147.18
147.01
146.88
146.80
146.80
a
b
c
d
Figure 1 a. XRD pattern of Mo2C-0.4, Mo2C-0.45, Mo2C-0.5, Mo2C-0.6 and MoC-1.0; b. XRD pattern of MoC-1.0 and MoC (PDF 65-280); c. XRD pattern of Mo2C-0.45, Mo2C-0.5, Mo2C-0.6 and Mo2C ( PDF 79-744); d. Shifts of XRD reflection of Mo2C-0.4 compared to Mo2C-0.45
(a) Figure 3 (a) The relation between trig-Mo2C and hex-Mo2C structure. (b) The relation between orthor-Mo2C
and
hex-Mo2C
structure.
The
coordinates in the light boxes and the unit cell parameters in the dash-lined boxes are ideal values calculated from hex-Mo2C assuming no distortions. Space groups are shown in full Herman-Mauguin symbol.
(b)
a
b
Figure 2 High temperature XRD of the formation process of Mo2C under N2 a. XRD reflection of the Mo2C precursor heated to 100 , 290 , 310 , 350 , 450 , 550 and 650 ; b. XRD reflection of the Mo2C precursor heated to 700 , 750 , 770 , 790 , 810 , 830 and 850
S0022-4596(19)30466-9
DOI:
https://doi.org/10.1016/j.jssc.2019.120961
Reference:
YJSSC 120961
To appear in:
Journal of Solid State Chemistry
Received Date: 23 November 2018 Revised Date:
12 September 2019
Accepted Date: 16 September 2019
Please cite this article as: Y. Tao, S. Zhu, Z. Pan, S. Qiu, X. Shen, Preparation and process investigation of molybdenum carbide and their N-doped analogue by calcination, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.120961. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Preparation and Process Investigation of Molybdenum Carbide and their N-doped Analogue by Calcination Yaqiu Taoa,b, Shuaishuai Zhua, Zhigang Pana,b *, Simin Qiua, Xiaodong Shena a
College of Materials Science and Engineering, Nanjing Tech University, 30 South Puzhu Road,
Nanjing, Jiangsu Province 21009, PR China b
State Key laboratory of Materials-Oriented Chemical Engineering, 30 South Puzhu Road, Nanjing,
Jiangsu Province 21009, PR China Corresponding author's email: [email protected] Abstract The crystal structure of Mo2C can be derived from close packed Mo atoms with partial occupation of C atoms in the octahedral interstices. Mo2C and MoC were successfully synthesized by calcinaiton of the lab-made
precursor
containing
1,8-diaminonaphthalene
and
ammonium
molybdate
at
carbon/molybdenum ratio of 2.0, 2.25, 2.7 and 4.5, respectively. High temperature XRD was used in the formation process study of Mo2C, which presents MoO2 as an important intermediate in formation of Mo2C. A reduction of the carbon/molybdenum ratio to 1.8 in the precursor caused shifts of XRD reflections in Mo2C-0.4. Rietveld refinement of the XRD reflections of Mo2C-0.4 reveals a partial nitrogen substitution of carbon atoms in Mo2C, which is confirmed by the theoretical calculation. The theoretical carbon and nitrogen contents of Mo2N0.6C0.4 agree with the experimental carbon and nitrogen content results of Mo2C-0.4 from element analysis. This confirms the substitution of carbon by nitrogen in the Mo2C lattice. Introduction Transition metal carbides are applicable in many cases due to their outstanding physicochemical properties and received more attention in the last decade than ever. Molybdenum carbides are found active in many catalytic applications. MoC presents promotion effects in Au and Pt catalyzed H2 production reactions1, 2. Mo2C nanowires and Mo2C modified carbon nano tube materials are synthesized by calcination in an inert atmosphere and used in methanol decomposition3-5. Mo2C/carbon composite materials are used in lithium storage6 and H2 generation from formic acid7. Mo2C catalysts also present high reactivity in both the hydrogen evolution reaction8, 9 and acetone hydrodeoxygenatin10. The oxidation of Mo2C to MoO2 is recognized as the main cause of the deactivation of this catalyst in methane reforming reactions11.
Besides the traditional temperature programmed reduction12-18 synthesis of Mo2C, a new thermal decomposition method of organic-inorganic hybrids is also studied in the past decade. Organic compounds including hexamethylenetetramine19-21, aniline22，glucose23 and biomass24 are used as carbon sources in the annealing preparation of Mo2C materials. A relatively close contact between Mo and ligands are presented in the molybdenum formamidinate complexes
25
, which provides a short
molybdenum carbon distance in the precursor for calcination. Both cubic (fcc) MoC and orthorhombic (hcp) Mo2C26 phase molybdenum carbides are found in the products of calcination. Carbon/metal ratio in the precursor plays an important role in modifying the crystal phase of the annealing products20, 22, 27. A 2D Mo2C material is prepared by chemical vapor deposition28, the STEM study of this material presents an orthorhombic structure. The MoC phase product is formed by calcination with a precursor at a higher carbon/metal ratio29. Heteratom including N and P doped Mo2C cause interests to more by their unique catalysis properties30. A nitrogen doped Mo2C is prepared by31 nitrogen rich ligands such as cetyltrimethyl ammonium bromide31,32. At temperatures higher than 1960℃, Mo2C possesses a high symmetry space group P63/mmc (No.194). In a P63/mmc unit cell, the two Mo atoms are located at the Wyckoff position 2c. The hexagonal structure is transformed to P 31m (No. 162) at 1350-1690℃. At temperatures lower than 1350℃, the trigoral structure transfers to Pbcn (No. 60). In this orthorhombic structure, Mo atoms are in general positions while the four carbon atoms are located in Wyckoff 4c with the coordinates (0,y,1/2), (1/2,-y+1/2,3/4), (0,-y,3/4) and (1/2,y+1/2,1/4) (where x = 0.375). Our previous study showed that Mo2C on active carbon present high selectivity and activity in H2 generation from formic acid decomposition7. The formation of appropriate molybdenum carbide phase is crucial in the catalytic activity study. In this work, a serial of molybdenum carbide sample was prepared by modifying the carbon/molybdenum ratio in the precursor for Mo2C or MoC phase synthesis. Mo2C dominate the calcinaiton products at the carbon/molybdenum ratio of 2.0, 2.25 and 2.7. A high temperature XRD study of this 1,8-Diaminonaphthalene contained molybdenum precursor reveals MoO221,
33
as an important intermediate in the formation of Mo2C. MoC is prepared by
calcination of the precursor containing excess carbon source. A further reduction of the carbon/molybdenum ratio to 1.8 in the precursor caused shifts of XRD reflections in Mo2C-0.4. Rietveld refinement of these XRD reflections of Mo2C-0.4 reveals a partial substitution of the carbon atoms by nitrogen in Mo2C. This is confirmed by theoretical calculation and elemental analysis of
Mo2C-0.4. Experiment Preparation of Molybdenum carbides and analogue The molybdenum carbides and analogue were synthesized via a temperature programmed calcination of lab-prepared precursor of molybdenum under N2. Ammonium molybdate was used as the molybdenum source and 1,8-diaminonaphhalene was used as the carbon source of these carbides. An aqueous solution of ammonium molybdate was added dropwise to the ethanol solution of 1,8-aminonaphhalene at pH=1. The remainder obtained by drying the mixed solution was calcinated under N2 at 750
for 5h to prepare the desired carbide.
Characterization Methods All the prepared carbides were analyzed by N2 adsorption/desorption using a ASPS2020 accelerated surface area porosimetry from Micromeritics. The specific areas were determined by applying the Brunauer-Emmet-Teller (BET) method in the relative pressure p/p0 range between 0.05 and 0.3. Power X-ray diffraction was carried out on a Smartlab 9Kw diffractionmetre in reflection geometry (Ni-Filtered, λ = 1.5418 Å). A heating rate of 20℃/min were used in the high temperature XRD spectra study under N2. The XRD reflection of the calcination product were detected after heated to a set temperature and remained at the same temperature till the end of the XRD signal collection. The morphologies study was performed on a JEOL LSM-6510 scanning electron microscope (SEM) coupled with a EDX detector from Thermo Scientific. In the Rietveld refinement, the orthor-Mo2C crystal structure was taken as the initial model. Only the phase scale factor and six coefficients of a shifted-Chebyshev polynomial to approximate the background were refined. Lattice parameters, peak shape parameters, grain-size peak broadening were then allowed to refine. No atomic parameter was refined. Attempts to refine the isotropic displacement parameters lead to non-meaningful values and therefore were not included in the final Rietveld refinement. The first principle calculations of the structural parameters of N doped Mo2C have been performed using the Cambridge Serial Total Energy Package (CASTEP) employing the periodic density functional theory. The Generalized Gradient Approximation (GGA) using the PBE (Perdew–Burke–Ernzerhof) functional was chosen to account for Exchange–correlation effects. The ionic cores were described using ultra-soft pseudopotentials, with a plane wave cutoff of 360 eV. For this calculation, a Monkhorst-Pack scheme has been used to sample the Brillouin zone; a 3 × 3 × 3 k-point mesh size was employed to properly describe bulk Mo2C. Tolerances for energy convergence and forces were set at
5x10-4 and 0.01 eV/Å, respectively34. Result and discussion XRD Characterization and SEM imaging As described in the experimental section, the carbon/molybdenum ratio of the precursor were carefully controlled, which results in different crystal phase. The carbon/molybdenum ratio of 1.8:1, 2.0:1, 2.25:1, 2.7:1 and 4.5:1 are used and the products obtained are labeled as Mo2C-0.4, Mo2C-0.45, Mo2C-0.5, Mo2C-0.6 and MoC-1.0. The XRD patterns of the products obtained from different carbon/molybdenum ratio are presented in Figure 1a. All the XRD patterns of the products obtained from calcination are similar except the high carbon/molybdenum precursor produced a different molybdenum carbide phase. Compared to the database, the XRD pattern of MoC-1.0 presents 2θ signals of 36.6º, 42.38º, 61.64º, 73.45º and 77.60º, which is consistent with MoC crystal phase, shown in Figure 1b. Nano sized MoC crystalline was formed from calcination. With a lower carbon/molybdenum ratio at 2.0:1, 2.25:1and 2.7:1, the Mo2C crystalline phase was observed from the XRD pattern, Figure 1c. The XRD signals are slightly broadened compared to the reference (PDF79-744 ). Though the XRD pattern of Mo2C-0.4 is similar to that of Mo2C-0.5, a close inspect of the XRD data reveals shifts of several signals at 1.8:1 carbon/molybdenum ratio, shown in Figure2 d. The decrease of carbon source caused the changes in d-spacing, as indicated by the shifts of the XRD reflections. The structure refinement of this compounds is discussed in detail in the follow text. High temperature XRD A high temperature XRD under N2 is used in the forming process study of molybdenum carbide from the precursor. A platinum plates was used as the heating plates and XRD testing substrate. When heated to 290℃, a hexagonal MoO3 phase was formed as presented in Figure 2a. The pyrolsis product remained as hexagonal phase MoO3 until the precursor was heated to 450℃. At 450℃, a phase transfer of hexagonal MoO3 to orthorhombic MoO3 was detected and some molybdenum oxide with a formula of MoO2-3 intermediate phase is also formed which indicated a losing of oxygen atom in the molybdenum oxide lattice and a reduction of Mo(III) to Mo(II). A further increase of the intermediate MoO2-3 phase is detected in the XRD pattern at 550℃ with a reduction of the orthorhombic MoO3 phase. A further treatment to 650℃ and 700℃, the precursor is converted to a relatively pure MoO2 phase,
which is a main intermediate phase to form Mo2C. The characteristic XRD signal of Mo2C at 2θ=39.5º was first observed at 750℃, and the intensity of this signal kept increasing till the end of the high temperature XRD study to 850℃, shown in Figure 2b. The high temperature XRD study reveals MoO2 as an important intermediate in the formation of Mo2C phase. Mo(III)
in the precursor was first
reduced to Mo(II) before converted to Mo2C. Organic carbon source, eg 1,8-Diaminonaphthalene, does not only contribute as the carbon source in these carbides but also as a reduction agent in converting Mo(III) to Mo(II) and Mo2C. Structure refinement of Mo2C-0.4 and theoretical calculation The symmetry reduction from P63/mmc to P 31m involves two steps. At first, the hexagonal symmetry is lost leading to a trigonal subgroup
3 1. The Wyckoff position 2a of the carbon atoms in P63/mmc
splits into two independent Wyckoff positions 1a and 1b in 3 1. In the second step, the subgroup
P 31m is obtained with a triple-sized unit cell. The Wyckoff position 1a in 3 1 splits into the Wyckoff positions 1a and 2c in P 31m while the Wyckoff position 1b splits into the Wyckoff positions 1b and 2d. As shown in Fig.3a, when the hex-Mo2C structure is transformed to the trig-Mo2C structure, the lattice parameters changes slightly and the fractional coordinates of all atoms remain the same while the occupancies for carbon atoms are varied. The symmetry reduction from P63/mmc to Pbcn involves three steps, In the first step, the hexagonal symmetry is lost resulting in an orthorhombic subgroup with a double sized C-centered cell. The centering is then removed in the second step and half of the translations is lost. In the third step, the a-axis is doubled and the orthorhombic subgroup Pbcn is obtained with interchange of axes. As a consequence, the corresponding Wyckoff position 2a of the carbon atoms in P63/mmc splits into two independent Wyckoff positions 4c in Pbcn, one of which is occupied by carbon atoms and the others are vacant, as shown in Figure.3b. In the synthesis of Mo2C and analogues, the metal/carbon ratio of the precursor were carefully controlled to obtain the desired products. As described previously the carbon/molybdenum ratio were managed from 1.8:1 to 4.5:1, which resulted in different products though the calcination were carried out under the same condition. A Mo2C phase material was successfully synthesized at a carbon/molybdenum ratio at 2.0:1, 2.25:1 and 2.7:1 as presented in the XRD spectra in Figure 1c. The Rietveld refinement for Mo2C-0.45 using generalized structure analysis system (GSAS) presents an orthorhombic unit cell where a=4.74Å, b=6.02Å and c=5.21Å, with a unit cell volume of 148.67 Å3, as
listed in Table 1. A refinement of the XRD data of Mo2C-0.4 , reveals a unit cell where a=4.78Å, b=5.96Å and c=5.15Å, with a unit cell volume of 146.72 Å3. As shown in Figure 1d, besides the unmoved signal at 2θ=52.1º, the unit cell distortion of Mo2C-0.4 cause a XRD signal shifts to lower angel at 2θ=37.8 º and 69.5º and higher angel for signals at 2θ=34.3º, 39.8º, 61.5º, 74.5º and 75.6º. In the geometric optimization by first principle calculation, the orthorhombic cell of Mo2C with a space group of Pbcn was used as the model. Nitrogen atom was used to substitute the carbon atom in the octahedron structure formed by molybdenum and carbon in the lattice. A set of model with the formula of Mo2C1~xNx (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1) were constructed and used in the structure refinements. As shown in Table 2, with the increase amount of N in the structure, the a-axis increases while both the b- and c-axis decreases with the volume of the unit cell. This results shows a consistency with that of the Rietveld refinements listed in Table 1. Take all these parameters into account, Mo2C-0.4 is consistent with the structure in which 60-70% carbon atoms substituted by N. The theoretical carbon and nitrogen contents of Mo2N0.6C0.4 are 2.3% and 4.0%, respectively. This result is agreed with the carbon and nitrogen contents results obtained from element analysis of Mo2C-0.4, shown in supporting information Table 1. Conclusion Mo2C and MoC were successfully synthesized by calcinaiton from the lab-made precursor of 1,8-diaminonaphthalene and ammonium molybdate at a carbon/molybdenum ratio of 2.0, 2.25 and 2.7 and 4.5, respectively. High temperature XRD was used in the formation process study of Mo2C; MoO2 is an important intermediate in formation of Mo2C. A reduction of the carbon/molybdenum ratio to 1.8 in the precursor caused shifts of XRD signals in Mo2C-0.4. Rietveld refinement of these XRD reflections of Mo2C-0.4 reveals the partial nitrogen substitution of carbon atoms in Mo2C, which is confirmed by theoretical calculation. The theoretical carbon and nitrogen contents of Mo2N0.6C0.4 is agreed with the carbon and nitrogen contents obtained from element analysis of Mo2C-0.4. This confirms the substitution of carbon by nitrogen in the Mo2C lattice. Acknowledgements We sincerely appreciate the financial support of grant from the Natural Science Funds for Young Scholar of China (Grant No.21107049) and the priority academic program development of Jiangsu Higher Education Institution (PAPD).
Reference 1.
Low-temperature hydrogen production from water and methanol using Pt/α-MoC catalystsL. Lin, W. Zhou, R. Gao, S. Yao, X. Zhang, W. Xu, S. Zheng, Z. Jiang, Q. Yu, Y.-W. Li, C. Shi, X.-D. Wen and D. Ma, Nature, 2017, 544, 80-83.
2.
Atomic-layered Au clusters on α-MoC as catalysts for the low-temperature water-gas shift reactionS. Yao, X. Zhang, W. Zhou, R. Gao, W. Xu, Y. Ye, L. Lin, X. Wen, P. Liu, B. Chen, E. Crumlin, J. Guo, Z. Zuo, W. Li, J. Xie, L. Lu, C. J. Kiely, L. Gu, C. Shi, J. A. Rodriguez and D. Ma, Science 2017, 357, 389-393.
3.
Synthesis of Nanoporous Molybdenum Carbide Nanowires Based on Organic-Inorganic Hybrid Nanocomposites with Sub-Nanometer Periodic StructuresQ. Gao, C. Zhang, S. Xie, W. Hua, Y. Zhang, N. Ren, H. Xu and Y. Tang, Chemistry of Materials, 2009, 21, 5560-5562.
4.
Preparation of supported Mo2C-based catalysts from organic–inorganic hybrid precursor for hydrogen production from methanol decomposition Q. Gao, C. Zhang, S. Wang, W. Shen, Y. Zhang, H. Xu and Y. Tang, Chem Commun (Camb), 2010, 46, 6494-6496.
5.
A nanoporous molybdenum carbide nanowire as an electrocatalyst for hydrogen evolution reaction L. Liao, S. Wang, J. Xiao, X. Bian, Y. Zhang, M. D. Scanlon, X. Hu, Y. Tang, B. Liu and H. H. Girault, Energy Environ. Sci., 2014, 7, 387-392.
6.
Synthesis, characterization and lithium-storage performance of MoO2/carbon hybrid nanowires Q. Gao, L. Yang, X. Lu, J. Mao, Y. Zhang, Y. Wu and Y. Tang, J. Mater. Chem., 2010, 20, 2807-2812.
7.
Large volume in situ H2 production on fixed bed reactor by concentrated formic acid aqueous solution Y. Tao, L. Tao, Z. Pan, S. Qiu and X. Shen, Fuel, 2018, 217, 106-110.
8.
3D Hierarchical Porous Mo 2 C for Effi cient Hydrogen EvolutionH. Ang, H. Wang, B. Li, Y. Zong, X. Wang and Q. Yan, Small, 2016, 12, 2859-2865.
9.
Mo2C Nanoparticles Dispersed on Hierarchical Carbon Microflowers for Efficient Electrocatalytic Hydrogen Evolution Y. Huang, Q. Gong, X. Song, K. Feng, K. Nie, F. Zhao, Y. Wang, M. Zeng, J. Zhong and Y. Li, ACS nano, 2016, 10, 11337-11343.
10.
Acetone Hydrodeoxygenation over Bifunctional Metallic−Acidic Molybdenum Carbide CatalystsM. M. Sullivan and A. Bhan, ACS Catalysis, 2016, 6, 1145-1152.
11.
Insights into the deactivation mechanism of metal carbide catalysts for dry reforming of methane via comparison of nickel-modified molybdenum and tungsten carbidesZ. Yao, J. Jiang, Y. Zhao, F. Luan, J. Zhu, Y. Shi, H. Gao and H. Wang, RSC Adv., 2016, 6, 19944-19951.
12.
S. Tuomi, R. Guil-Lopez and T. Kallio, Journal of Catalysis, 2016, 334, 102-109.
13.
Acid site densities and reactivity of oxygen-modified transition metal carbide catalystsM. M. Sullivan and A. Bhan, Journal of Catalysis, 2016, 344, 53-58.
14.
Low temperature CO2 hydrogenation to alcohols and hydrocarbons over Mo2C supported
15.
Cu-Mo2C/MCM-41: An Efficient Catalyst for the Selective Synthesis of Methanol from CO2 X.
metal catalystsY. Chen, S. Choi and L. T. Thompson, Journal of Catalysis, 2016, 343, 147-156. Liu, Y. Song, W. Geng, H. Li, L. Xiao and W. Wu, Catalysts, 2016, 6, 75. 16.
Reaction intermediate species during the steam reforming of methanol over metal modified molybdenum carbide catalystsJ. Cao, Y. Ma, G. Guan, X. Hao, X. Ma, Z. Wang, K. Kusakabe and A. Abudula, Applied Catalysis B: Environmental, 2016, 189, 12-18.
17.
R. Li, A. Shahbazi, L. Wang, B. Zhang, A. M. Hung and D. C. Dayton, Applied Catalysis A:
General, 2016, 528, 123-130. 18.
Mo2C catalyzed vapor phase hydrodeoxygenation of lignin-derived phenolic compound mixtures to aromatics under ambient pressureC.-J. Chen, W.-S. Lee and A. Bhan, Applied Catalysis A: General, 2016, 510, 42-48.
19.
One-step synthesis of bulk Mo and Ni–Mo carbides for methanation X. Huo, Z. Wang, J. Huang, R. Zhang and Y. Fang, RSC Adv., 2016, 6, 24353-24360.
20.
Bulk Mo and Co–Mo carbides as catalysts for methanation X. Huo, Z. Wang, J. Huang, R.
21.
The efficient synthesis of a molybdenum carbide catalyst via H2-thermal treatment of a
Zhang and Y. Fang, Catalysis Communications, 2016, 79, 39-44. Mo(VI)–hexamethylenetetramine complex Z.-Q. Wang, Z.-B. Zhang and M.-H. Zhang, Dalton Trans., 2011, 40, 1098-1104. 22.
Controllable synthesis of α-MoC1-x and β-Mo2C nanowires for highly selective CO2 reduction to CO J. Gao, Y. Wu, C. Jia, Z. Zhong, F. Gao, Y. Yang and B. Liu, Catalysis Communications, 2016, 84, 147-150.
23.
Ni/b-Mo2C as noble-metal-free anodic electrocatalyst of microbial fuel cell based on Klebsiella pneumoniae L. Z. Zeng, S. F. Zhao, Y. Q. Wang, H. Li and W. S. Li, Int. J. Hydrogen Energy, 2012, 37, 4590-4596.
24.
Selective hydrogenation of lignin to produce chemical commodities by using a biochar supported Ni–Mo2C catalyst obtained from biomassY.-Y. Wang, L.-L. Ling and H. Jiang, Green Chem., 2016, 18, 4032-4041.
25.
Dimolybdenum complexes with mixed formamidinate ligands Y.-Y. Wu, Y.-C. Kao, J.-D. Chen and C.-H. Hung, Inorganica Chimica Acta, 2004, 357, 1002-1010.
26.
Performances of mixed alcohols synthesis over potassium promoted molybdenum carbides M. Xiang, D. Li, W. Li, B. Zhong and Y. Sun, Fuel, 2006, 85, 2662-2665.
27.
An expeditious synthesis of early transition metal carbide nanoparticles on graphitic carbonsD. Ressnig, S. Moldovan, O. Ersen, P. Beaunier, D. Portehault, C. Sanchez and S. Carenco, Chemical communications, 2016, 52, 9546-9549.
28.
Unique Domain Structure of Two-Dimensional α‑Mo2C Superconducting CrystalsZ. Liu, C. Xu, N. Kang, L. Wang, Y. Jiang, J. Du, Y. Liu, X. L. Ma, H. M. Cheng and W. Ren, Nano letters, 2016, 16, 4243-4250.
29.
Synthesis of a-MoC1-x Nanoparticles with aSurface-Modified SBA-15 Hard Template: Determination of Structure–FunctionRelationships in Acetic Acid Deoxygenation F. G. Baddour, C. P. Nash, J. A. Schaidle and D. A. Ruddy, Angewandte Chemie, 2016, 55, 9026-9029.
30.
Pomegranate-like N,P-Doped Mo2C@C Nanospheres as Highly Active Electrocatalysts for Alkaline Hydrogen Evolution Y. Y. Chen, Y. Zhang, W. J. Jiang, X. Zhang, Z. Dai, L. J. Wan and J. S. Hu, ACS nano, 2016, 10, 8851-8860.
31.
Ultra-efficient electrocatalytic hydrogen evolution at one-step carbonization generated molybdenum carbide nanosheets/N-doped carbon C. Du, H. Huang, Y. Wu, S. Wu and W. Song, Nanoscale, 2016, 8, 16251-16258.
32.
Mo2C quantum dot embedded chitosan-derived nitrogen-doped carbon for efficient hydrogen evolution in a broad pH range Z. Pu, M. Wang, Z. Kou, I. S. Amiinu and S. Mu, Chemical communications, 2016, 52, 12753-12756.
33.
Hydrogen production by steam reforming of biomass tar over biomass char supported
molybdenum carbide catalyst M. Kaewpanha, G. Guan, Y. Ma, X. Hao, Z. Zhang, P. Reubroychareon, K. Kusakabe and A. Abudula, Int. J. Hydrogen Energy, 2015, 40, 7974-7982. 34.
Materials Studio 6.1; Accelrys, Inc., 2012.
Table 1 GSAS refinement of the Mo2C-0.4 and Mo2C-0.45 Lattice parameter
Mo2C-0.45
Mo2C-0.4
a/ Å
4.74
4.78
b/ Å
6.02
5.96
c/ Å
5.21
5.15
V/ Å3
148.67
146.72
Table 2 Theoretical calculation of N doped Mo2C structure Lattice parameter
Mo2C
Mo2N0.1C0.9 Mo2N0.2C0.8 Mo2N0.3C0.7 Mo2N0.4C0.6 Mo2N0.5C0.5 Mo2N0.6C0.4 Mo2N0.7C0.3 Mo2N0.8C0.2 Mo2N0.9C0.1
Mo2N
a/ Å
4.74
4.74
4.74
4.74
4.75
4.76
4.77
4.78
4.80
4.83
4.87
b/ Å
6.04
6.04
6.04
6.03
6.03
6.02
6.02
6.01
6.00
5.98
5.97
c/ Å
5.22
5.20
5.19
5.17
5.16
5.14
5.13
5.12
5.10
5.08
5.06
V/ Å3
149.36
148.87
148.43
148.03
147.69
147.40
147.18
147.01
146.88
146.80
146.80
a
b
c
d
Figure 1 a. XRD pattern of Mo2C-0.4, Mo2C-0.45, Mo2C-0.5, Mo2C-0.6 and MoC-1.0; b. XRD pattern of MoC-1.0 and MoC (PDF 65-280); c. XRD pattern of Mo2C-0.45, Mo2C-0.5, Mo2C-0.6 and Mo2C ( PDF 79-744); d. Shifts of XRD reflection of Mo2C-0.4 compared to Mo2C-0.45
(a) Figure 3 (a) The relation between trig-Mo2C and hex-Mo2C structure. (b) The relation between orthor-Mo2C
and
hex-Mo2C
structure.
The
coordinates in the light boxes and the unit cell parameters in the dash-lined boxes are ideal values calculated from hex-Mo2C assuming no distortions. Space groups are shown in full Herman-Mauguin symbol.
(b)
a
b
Figure 2 High temperature XRD of the formation process of Mo2C under N2 a. XRD reflection of the Mo2C precursor heated to 100 , 290 , 310 , 350 , 450 , 550 and 650 ; b. XRD reflection of the Mo2C precursor heated to 700 , 750 , 770 , 790 , 810 , 830 and 850