Preparation of Mo2C by the temperature-programmed reaction between h-MoO3 and CO

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 + MoO_xC_y→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.     

Study on reduction reaction of MoO2 powder with NH3

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, MoO_xN_1?x as an intermediate product is formed during the reduction processes.     

Study on the reduction of commercial MoO3 with carbon black to prepare MoO2 and Mo2C nanoparticles

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.     

Phase equilibrium diagram and phase transformation for preparation of Mo2C: Thermodynamic study and experimental verification

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.     

Ultra‐low Sintering Temperature Microwave Dielectric Ceramics Based on Na2O‐MoO3 Binary System

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 (ε_r) of 4.1–12.9, the Q × f values of 19900–62400 GHz, and the τ_f 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 Na^I‐O and Mo^VI‐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.     

Mo2C intermediate layers for the wetting and infiltration of graphite foams by liquid copper

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.     

Mo+TiO2(110) Mixed Oxide Layer: Structure and Reactivity

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.     

Influence of alkali metal (Li and Cs) addition to Mo2N catalyst for CO hydrogenation to hydrocarbons and oxygenates

The aim of this work is to understand the catalytic behaviour of Li and Cs promoted Mo₂N for CO hydrogenation to hydrocarbons and oxygenates at the reaction conditions 275–325 °C, 7 MPa, and 30 000 h^?1 GHSV. Molybdenum nitrides were synthesized via temperature programmed treatment of ammonium heptamolybdate (AHM) and alkali metal (AM) precursors under continuous gaseous ammonia flow. Unpromoted Mo₂N and AM‐Mo₂N catalysts were characterized using BET‐pore size, X‐ray diffraction, TPD‐mass of CO, HR‐TEM, and XPS techniques. Nominal loadings of 1, 5, and 10 wt% of Li and Cs were selected for these studies. At a 10 % CO conversion level, the total oxygenate selectivity of 28, 11, and 6.5 % was observed on 5Cs‐Mo₂N, 5Li‐Mo₂N, and unpromoted Mo₂N, respectively. The decreased oxygenate selectivity for unpromoted Mo₂N was mainly associated with CO dissociative hydrogenation on Mo^δ+ sites. On the other hand, improved molecular CO insertion into ?C_xH_y intermediate accelerates the total oxygenate formation on the Cs‐Mo‐N catalyst. However, during nitridation, crystal structure changes were observed in Li‐Mo‐N and the obtained oxygenates selectivity was attributed to the Li₂MoO₄ phases. At lower AM loadings, the active sites corresponding to oxygenates formation were inadequate, and at higher AM loadings, surface metallic molybdenum decreased the total oxygenate selectivity.

     

Kinetic study of carbon nanotube synthesis over Mo/Co/MgO catalysts

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.     

Different Catalytic Reactions of n-Hexane and 1-Hexene on Molybdenum Based Catalysts

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(H_x)_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.     

The reduction behavior of Mo/TiO2-Al2O3 catalyst

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.     

Conversion of methane to benzene over Mo2C and Mo2C/ZSM-5 catalysts

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.     

Remarkable support effect of ZrO2 upon the CO2 reforming of CH4 over supported molybdenum carbide catalysts

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 MoC_1−x (x≈0.5), prepared through nitriding before carburization. The conversion levels over MoC_1−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 MoC_1−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.     

Preparation, characterization, and catalytic properties of the Mo2C/SBA-15 catalysts

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.     

Synthesis, Characterization and Catalytic Activity in the Hydrogenation of Cyclohexene with Molybdenum Carbide

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.     

Nucleation and growth of graphene/Mo2C heterostructures on Cu through CVD

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.     

In situ time-resolved characterization of novel Cu–MoO2 catalysts during the water–gas shift reaction

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.     

The Effect of ppm-Level Na Impurity on the Structure of SiO2-Supported Mo Catalysts Prepared in a Clean Room

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.     

Electrode Performances of Amorphous Molybdenum Oxides of Different Molybdenum Valence for Lithium-ion Batteries

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.     

Decomposition of metal carbides as an elementary step of carbon nanotube synthesis

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 MoC_1−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.