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1.
《Ceramics International》2023,49(20):33135-33146
In the work, the temperature-programmed reaction (TPR) between hexagonal-shaped h-MoO3 and high-purity CO under different heating rates was investigated in order to prepare Mo2C. Various technologies such as TG-DTA-DTG, XRD, FESEM, FT-IR and Raman spectrum as well as the thermodynamic calculation were adopted to analyze the experimental data. The results showed that the physically adsorbed water on the sample surface, the residual ammonium and coordinated water in the internal structure of h-MoO3 were successively released as the temperature increased, and then α-MoO3 and Mo4O11 were formed when the temperature arrived at around 791 K. Upon further increasing the temperature, the reduction process occurred and MoO2 will be generated. Thereafter, the carburization reaction was taken place and the subsequent reaction pathways were significantly different at lower and higher heating rates: at lower heating rates (8 and 12 K/min), the carburization process of MoO2 to Mo2C followed MoO2→MoO2+Mo2C→Mo2C + Mo→Mo2C; while at higher heating rates (16 and 20 K/min), the reaction pathways followed MoO2→MoO2+Mo2C→MoO2+Mo2C + Mo + MoOxCy→Mo→Mo2C, single-phase metallic Mo can be generated. The work also discovered that the as-prepared Mo2C always kept the same platelet-shaped morphology as that of the newly-formed MoO2; while due to the removal of oxygen and the decrease of molar volume during the transformation process, the as-prepared Mo2C exhibited a rougher and more porous morphological structure. 相似文献
2.
Reaction mechanism between MoO2 powder and NH3 in the temperature range of 1028‐1273 K has been investigated. The results show that reduction products are strongly dependent on the reaction temperature employed. It is found that molybdenum nitride (Mo2N) can be successfully synthesized when the temperature is below 1078 K, with the morphologies keep the same as that of the raw material MoO2. Although the temperature is above 1078 K, metal Mo powder will be turned up; at the even higher temperatures, the thermodynamically stable phase will be metal Mo. In addition, MoOxN1?x as an intermediate product is formed during the reduction processes. 相似文献
3.
Guo-Dong Sun Guo-Hua Zhang Bai-Jun Yan Kuo-Chih Chou 《International Journal of Applied Ceramic Technology》2020,17(3):917-931
A simple method was developed to synthesize MoO2 and Mo2C nanoparticles via controlling nucleation and growth in carbothermic reduction of commercial MoO3 with carbon black. It was found that the appropriate C/MoO3 molar ratio for preparation of Mo2C was 2.8, and the carbothermic reduction process followed the sequence: MoO3 → transport phase (TP) → MoO2 → Mo2C. It was revealed that the most crucial issues for controlling number of produced particles of product were migration of Mo source and aid of nucleating agent, which can be achieved by using MoO3 and carbon black as starting materials. MoO2 nanosheets with the thickness of 12 nm and lateral size of 60 nm, as well as Mo2C nanoparticles with particle size of 30 nm were prepared via reduction of MoO3 with carbon black. However, MoO2 and Mo2C produced via reducing MoO3 by other kinds of carbon sources (activated carbon, graphite) or gas reductants (10% CH4/H2, CO) had much larger particle sizes of a few micrometers, which were tens of times than those using MoO3 and carbon black, due to the too small amount of formed nuclei. The effects of C/MoO3 molar ratio (0.5-2.8), molybdenum sources and carbon sources on the reaction mechanisms were investigated in detail. 相似文献
4.
《Ceramics International》2020,46(1):755-762
Because of the extended application in catalytic reactions, numerous kinds of methods are developed for synthesis of Mo2C catalysts. The repeated efforts on exploring the reaction conditions caused a huge waste of materials and a large consumption of the energy from researchers. In this work, a widely applicable guidance of phase equilibrium diagrams for Mo2C production was provided by thermodynamic calculation. The phase equilibrium diagrams of both MoO3–C and MoO3–Na2CO3–C (Na2MoO4–C) systems were plotted, which reveals the reaction mechanisms and suggests the reaction conditions for preparation of Mo2C. According to the phase equilibrium diagrams of Na2MoO4–C system, the reaction routes of molybdenum determined by the reduction temperature will experience Na2MoO4, Mo2C, Mo in carbothermic reduction procedure, which is instructive for synthesis of Mo2C catalysts. Results of this work will provide a theoretic guidance for the generation of Mo2C, which will make the synthesis of molybdenum carbides much easier. 相似文献
5.
Ultra‐low Sintering Temperature Microwave Dielectric Ceramics Based on Na2O‐MoO3 Binary System
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Gao‐qun Zhang Hong Wang Jing Guo Li He Dan‐dan Wei Qi‐bin Yuan 《Journal of the American Ceramic Society》2015,98(2):528-533
The compounds in Na2O‐MoO3 system were prepared by the solid‐state reaction route. The phase composition, crystal structures, microstructures, and microwave dielectric properties of the compounds have been investigated. This series of compounds can be sintered well at ultra‐low temperatures of 505°C–660°C. The sintered samples exhibit good microwave dielectric properties, with the relative permittivities (εr) of 4.1–12.9, the Q × f values of 19900–62400 GHz, and the τf values of ?115 ppm/°C to ?57 ppm/°C. Among the eight compounds in this binary system, three kinds of single‐phase ceramics, namely Na2MoO4, Na2Mo2O7 and Na6Mo11O36 were formed. Furthermore, the relationship between the structure and the microwave dielectric properties in this system has been discussed. The average NaI‐O and MoVI‐O bond valences have an influence on the sintering temperatures in Na2O‐MoO3 system. The large valence deviations of Na and Mo lead to a large temperature coefficient of resonant frequency. The X‐ray diffraction and backscattered electron image results show that Na2MoO4 doesn't react with Ag and Al at 660°C. Also, Na2Mo2O7 has a chemical compatibility with Al at 575°C. 相似文献
6.
Since copper does not wet graphite foams (GFs), a method for the synthesis of Mo2C coatings throughout the GFs was developed for improving the wetting between GFs and copper and for the preparation of GF/copper composites. The coatings were formed on the GFs in a reaction medium consisting of ammonium paramolybdate, which decomposed to fine MoO3, in a mixture of molten NaCl–KCl salts. The formation mechanism and microstructure of the Mo2C coatings on GFs were investigated. Then the microstructure, thermal conductivity and thermal expansion behavior of the obtained GF/copper composites were studied. Results indicated the formation of Mo2C coatings occurred in two steps, namely, the reduction of MoO3 to MoO2 and the reduction of MoO2 to Mo2C. Copper was infiltrated into the Mo2C-coated GFs without external pressure and the nearly pore-free GF/copper composites were obtained. The thermal conductivity of the composite with a density of 5.76 g/cm3 reached 268.4 W/mK. Significant reduction in coefficient of thermal expansion of the composite compared with that of copper (8.91 versus 18.59 ppm/K) was obtained. 相似文献
7.
Osman Karslıoğlu Xin Song Helmut Kuhlenbeck Hans‐Joachim Freund 《Topics in Catalysis》2013,56(15-17):1389-1403
We present a STM/XPS/TPD/LEED study of the structural and electronic properties of Mo+Ti mixed oxide layers on TiO2(110), and of their interaction with water, methanol and ethanol. Several different preparation procedures were tested and layers with different degrees of Mo/Ti mixing were prepared. Ordered mixed oxide surface phases with distinct LEED patterns could not be found; for all investigated Mo concentrations a TiO2(110) like pattern was observed. Mo tends to agglomerate on the surface where it is found predominantly as Mo6+ at low coverages and as Mo4+ at high coverages. Mo4+ was also identified in the bulk of the mixed oxide layer. The Mo3d binding energies categorize the Mo4+ species as being dimeric. A third Mo3d doublet is attributed to a Mo species (Mo n+) with an oxidation state between those reported for Mo in MoO2 and metallic Mo. Two types of Mo-induced features could be identified in the STM images for low Mo concentrations (in the range of 1 %). At higher Mo concentrations (~50 %) the surface is characterized by stripes with limited lengths in [001] direction. The concentration of bridging oxygen vacancies, which are common defects on TiO2(110), is reduced significantly even at low Mo concentrations. Methanol and ethanol TPD spectra reflect this effect by a decrease of the intensity of the features related to these surface defects. At elevated MoO x coverages, the yield of reaction products in methanol and ethanol TPD spectra are somewhat smaller than those found for clean TiO2(110) and the reactions occur at lower temperature. 相似文献
8.
Influence of alkali metal (Li and Cs) addition to Mo2N catalyst for CO hydrogenation to hydrocarbons and oxygenates
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Sharif F. Zaman Nagaraju Pasupulety Abdulrahim A. Al‐Zahrani Muhammad A. Daous Hafedh Driss Saad S. Al‐Shahrani Lachezar Petrov 《加拿大化工杂志》2018,96(8):1770-1779
9.
The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH4 over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH4, as well as to the formation of Mo2C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH4 pressure, indicating that the dissociation of CH4 is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH4. 相似文献
10.
Controlled reduction by hydrogen, of the equivalent five monolayer of MoO3 deposited on TiO2 as a function of the reduction temperature up to 873 K, enabled us to obtain three Mo2O5, bifunctional (metal-acid) MoO2(Hx)ac and the metallic Mo(0) phases. Characterization of these phases was made by employing surface XPS-UPS techniques in parallel with catalytic reactions. Hydroisomerization of n-hexane occurs on the bifunctional phase, while hydrogenation/ dehydrogenation and benzene formation were performed by the metallic Mo state. 相似文献
11.
Zhaobin Wei Shaocong Jiang Qin Xin Shishan Sheng Guoxing Xieng 《Catalysis Letters》1991,11(3-6):365-374
The effect of TiO2 modified Al2O3 surface on the reducibility of MoO3 has been studied by TPR and XPS. The results show that Mo6+ in Mo/TiO2-Al2O3 can be reduced to much lower valency, especially at low Mo loading. The influence of the calcination temperature on the reduction of Mo6+ on Al2O3 and TiO2-Al2O3 carriers is different. The data reveals that the reducibility of Mo6+ on Al2O3 slightly decreased, while that on TiO2-Al2O3 increased when the calcination temperature was raised. It is suggested that the stronger tetrahedral site of the Al2O3 surface was first occupied by TiO2 and main octahedral Mo6+ in polymeric species-; and a small crystalline MoO3 formed on TiO2-Al2O3, whereas the formation of tetrahedral Mo6+ species and Al2(MoO4)3 phase was inhibited. 相似文献
12.
The activation and dehydrogenation of CH2 on Mo2C and MO2C/ZSM-5 have been investigated under non-oxidizing conditions. Unsupported Mo2C exhibited very little activity towards methane decomposition at 973 K. The main reaction pathway was the decomposition of methane to give hydrogen and carbon with a trace amount of ethane. Mixing Mo2C with ZSM-5 support somewhat enhanced its catalytic activity, but did not change the products of the reaction. A dramatic change in the product formation occurred on partially oxidized Mo2C/ZSM-5 catalyst; besides some hydrocarbons benzene was produced with a selectivity of 70–80% at a conversion of 5–7%. Carburization of highly dispersed MoO3 on ZSM-5 also led to a very active catalyst: the conversion of methane at the steady state was 5–6% and the selectivity of benzene formation was 85%.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged. 相似文献
13.
In reforming of CH4 with CO2 over molybdenum carbide catalysts, the catalytic performance of unsupported hexagonal Mo2C prepared by direct carburization of MoO3 was considerably different from a similar composition, cubic MoC1−x (x≈0.5), prepared through nitriding before carburization. The conversion levels over MoC1−x were substantially higher than those over Mo2C, although the turnover frequencies were lower. X‐ray diffraction analysis indicated that Mo2C deactivated by conversion to MoO2 during the reaction, but the MoC1−x was transformed to the hexagonal Mo2C and remained stable. The activity of Mo2C dispersed on various supports for the CH4–CO2 reaction was also investigated. The performance depended strongly on the property of supports, with the ZrO2‐supported Mo2C catalyst exhibiting the highest activity and durability for this reaction. Moreover, deactivation of Mo2C/ZrO2 at ambient pressure was suppressed by decreasing the loading amount of Mo2C.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
14.
Ping-Yi Wu Sheng-Fu Ji Lin-Hua Hu Ji-Qin Zhu Cheng-Yue Li 《Journal of Porous Materials》2008,15(2):181-187
A series of Mo2C/SBA-15 catalysts with different Mo contents were prepared by temperature-programmed carburization (TPC). The materials obtained
and their oxide precursors (MoO3/SBA-15) were characterized by Nitrogen adsorption-desorption isotherms, X-ray diffraction (XRD), and Fourier transform-infrared
(FT-IR) spectroscopy. The activities of the catalysts for deep hydrodesulfurization (HDS) of thiophene were evaluated. The
results of N2 adsorption-desorption isotherms indicated that the surface area and pore diameter of the oxide precursors increase after
carburization. The XRD patterns show that Mo2C particles are highly dispersed in the SBA-15 ordered mesoporous. The test results show that Mo2C/SBA-15 catalysts have an excellent performance for the deep HDS under the lower temperature region. 相似文献
15.
Espinoza-Monjardín J. Cruz-Reyes M. Del Valle-Granados E. Flores-Aquino M. Avalos-Borja S. Fuentes-Moyado 《Catalysis Letters》2008,120(1-2):137-142
Two series of molybdenum carbides are prepared from MoO3 by temperature programmed reduction (TPR), differing in feed gas composition (20 and 40% CH4/H2). The heating ramp consists of two steps, one from ambient temperature to 973 K, at a rate of 10 K/min; the second from 973 K
up to the final temperature (1023, 1073, 1123, 1173 K), at a rate of 0.5 K/min. The Mo2C (hcp) phase is identified for the series prepared with 20% CH4/H2 at different temperatures, with surface areas between 20 and 27 m2/g. Also found is a mix of the MoC and Mo2C (hcp) phases for the series prepared with 40% CH4/H2 at temperatures above 1023 K, with surface areas between 9 and 19 m2/g. Both series of catalysts reach 100% conversion of cyclohexene in 5 h or less, with those catalysts prepared with a 40%
CH4/H2 gas mix reaching maximum conversion in the least time. Catalysts are compared to a commercial molybdenum carbide reagent as
a reference. 相似文献
16.
Furkan Turker Omer R. Caylan Goknur Buke 《Journal of the American Ceramic Society》2022,105(2):815-822
We investigated the chemical vapor deposition synthesis of Mo2C/graphene heterostructures on a partially wetted liquid copper surface, studied the morphology of resulting phases using electron and optical microscopy, and determined the rate-limiting step for the growth of Mo2C on graphene. The morphology of the Mo2C crystals varied from the center to the edge of the copper substrate because of the change in the Mo diffusion pathways owing to the variation in the thickness of the Cu substrate. Thin, hexagonal-shaped crystals of Mo2C were found in the central region, where Cu is the thickest. In addition, the growth pressure substantially affects the nucleation and growth kinetics of both Mo2C and graphene. At high pressures (750 Torr), the graphene layer fully covered the Cu surface and Mo2C crystals formed with a regular shape, while at low pressures (5 Torr), the nucleation of both domains was suppressed, leading to the evolution of Mo2C crystals with irregular shapes. The activation energy for the growth of Mo2C on graphene was calculated to be 3.76 ± 0.3 eV, and the diffusion of Mo to the Cu surface through uncovered Cu or graphene vacancies/defects was determined to be the rate-limiting step. 相似文献
17.
Wen Wen Liu Jing Michael G. White Nebojsa Marinkovic Jonathan C. Hanson José A. Rodriguez 《Catalysis Letters》2007,113(1-2):1-6
A novel and active Cu–MoO2 catalyst was synthesized by partial reduction of a precursor CuMoO4 mixed-metal oxide with CO or H2 at 200–250 °C. The phase transformations of Cu–MoO2 during H2 reduction and the water–gas shift reaction could be followed by in situ time resolved XRD techniques. During the reduction process the diffraction pattern of the CuMoO4 collapsed and the copper metal lines were observed on an amorphous material background that was assigned to molybdenum oxides.
During the first pass of water–gas shift (WGS) reaction, diffraction lines for Cu6Mo5O18 and MoO2 appeared around 350 °C and Cu6Mo5O18 was further transformed to Cu/MoO2 at higher temperature. During subsequent passes, significant WGS catalytic activity was observed with relatively stable plateaus
in product formation around 350, 400 and 500 °C. The interfacial interactions between Cu clusters and MoO2 increased the water–gas shift catalytic activities at 350 and 400 °C. 相似文献
18.
To investigate the effect of alkali impurity in Mo/SiO2 on the MoO
x
structure on SiO2 surfaces SiO2-supported Mo oxides were prepared with various amounts of Na ions in a class-1 clean bench with a laminar flow in a class-1000 clean room. The Na concentrations were varied in the range 0-5000 ppm, while the Mo loading on SiO2 was maintained at 0.7 wt%. The Mo-Na/SiO2 samples obtained were characterized by diffuse reflectance UV-visible and Raman spectroscopy. Three types of Mo species were identified: octahedral monooxo Mo monomer species, Na2Mo2O7 and MoO3. At less than 100 ppm Na both octahedral Mo monomers and MoO3 species were formed on SiO2. The MoO3 species was transformed to Na2Mo2O7 at 2000 ppm Na, where the Na ions interact directly with the Mo species on the surface. The octahedral monooxo Mo monomer species seems not to be influenced significantly by Na impurity. 相似文献
19.
Jihyun Jang Sang-Mo Kim Youngjin Kim Kern H. Park Jun H. Ku Ji Heon Ryu Seung M. Oh 《Israel journal of chemistry》2015,55(5):604-610
Amorphous molybdenum oxides are prepared by a reduction of Mo6+ aqueous solution with KBH4. Two molybdenum oxides that differ in the average Mo valence are obtained by changing the solution pH, and their electrode performance is compared with each other. The X-ray absorption spectroscopy and thermogravimetric analysis illustrate that the sample prepared at the higher pH (4.0) shows an Mo valence of +5.5, which is close to that for MoO3, whereas the Mo valence (+4.4) for the other sample, prepared at the lower pH (0.8), is close to that for MoO2. The latter electrode outperforms the former with respect to cycle retention. The former electrode is lithiated by a conversion reaction, which is the case for MoO3. This electrode exhibits a larger initial capacity, but poorer cycle performance, due to massive volume change and repeated metal-oxygen bond breaking/formation with cycling. The other electrode, the Mo valence, which is +4.4, is lithiated by an addition reaction, in which Li+ ions are stored in structural defects, such as vacancies and void spaces. The superior cycle performance of this electrode can be ascribed to the absence of massive volume change and bond breaking/formation. 相似文献
20.
The role of catalyst components in catalysts containing molybdenum, Mo/M/MgO (MNi, Co, and Fe), as well as Mo-free catalysts, M/MgO (MNi, Co, and Fe), for carbon nanotube (CNT) synthesis have been investigated by TEM, XRD, and Raman spectroscopy. CNT synthesis by the catalytic decomposition of CH4 over M/MgO catalysts can proceed at reaction temperatures higher than the decomposition temperature of the metal carbides (Ni3C, Co2C, and Fe3C), which indicates that carbon in the CNT originates from the graphitic carbon formed on the catalyst surface by the decomposition of metal carbides. For all catalysts containing Mo, thin CNT formation starts at an identical temperature of 923 K, corresponding to the decomposition temperature of MoC1−x into Mo2C. The significant effect of the addition of Mo is concerned with the formation of Mo2C in a catalyst particle during CNT synthesis at high reaction temperatures. The presence of a stable Mo2C phase leads to the formation of thin CNT with better crystallinity at high reaction temperatures. The role of Ni, Co, and Fe in the Mo/M/MgO catalysts is ascribed to the dissociation of CH4. 相似文献