Reduction Of MoO3 Research Articles

Boosting Monolayer Transition Metal Dichalcogenides Growth by Hydrogen-Free Ramping during Chemical Vapor Deposition.

The controlled vapor-phase synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) is essential for functional applications. While chemical vapor deposition (CVD) techniques have been successful for transition metal sulfides, extending these methods to selenides and tellurides often faces challenges due to uncertain roles of hydrogen (H2) in their synthesis. Using CVD growth of MoSe2 as an example, this study illustrates the role of a H2-free environment during temperature ramping in suppressing the reduction of MoO3, which promotes effective vaporization and selenization of the Mo precursor to form MoSe2 monolayers with excellent crystal quality. As-synthesized MoSe2 monolayer-based field-effect transistors show excellent carrier mobility of up to 20.9 cm2/(V·s) with an on-off ratio of 7 × 107. This approach can be extended to other TMDs, such as WSe2, MoTe2, and MoSe2/WSe2 in-plane heterostructures. Our work provides a rational and facile approach to reproducibly synthesize high-quality TMD monolayers, facilitating their translation from laboratory to manufacturing.

Sintering behavior and densification mechanism of MoOx targets obtained by hot pressing

MoOx targets for magnetron sputtering Mo–O films were obtained by vacuum hot pressing sintering. MoOx powder was obtained by hydrogen reduction of MoO3, and the O/Mo atomic ratio was determined by the k-value method. The sintering behavior and densification mechanism of the MoOx target were analyzed by exploring the influence of hot pressing sintering pressure, temperature, and holding time on the target's density, phase composition, and morphology. MoOx targets with different oxygen content were sintered using MoOx powder with O/Mo of 2.838, 2.679, and 2.494, respectively. The results reveal that complex reactions occur in the shrinkage stage of sintering. The content of Mo4O11 increases, the content of Mo9O26 and MoO2 decreases, and the oxygen content of the targets is positively correlated with the content of MoO2 in MoOx powder. Subsequently, Mo4O11 grain growth densifies the targets. Excessive sintering temperature and pressure will cause the decomposition of Mo4O11, resulting in a decrease in density. Finally, the MoO2.623 target with a density of 97.85 % was obtained by hot pressing sintering with MoO2.494 as raw material at 973 K, 40 MPa, and holding temperature for 60 min. Raman mapping shows the MoO2.623 target with 16.9 wt% MoO2 and 83.1 wt% Mo4O11, and the phase distribution was uniform.

Hierarchically Porous Iron Alloys with High Sintering Resistance During Cyclic Steam Oxidation and Hydrogen Reduction

AbstractHydrogen release/storage materials for iron/air batteries are fabricated as Fe‐16Mo and Fe‐24Ni (at.%) lattices via 3D‐extrusion printing of foamed inks containing oxide microparticles, followed by H2 reduction and sintering. A hierarchical open porosity is designed: (i) channels between walls created during printing, (ii) mesopores within walls, created during ink foaming, and (iii) micropores within ligaments between mesopores, created from partial sintering metal particles and gas escape during cycling. When subjected to H2/H2O redox cycling at 800 °C, the lattices of both compositions gradually undergo sintering. The Fe‐16Mo lattice remains fully redox‐active after 50 cycles unlike the Fe‐24Ni lattice, which loses 90% of its redox capacity after 30 cycles. The increased lifetime of the Fe‐16Mo lattice is attributed to the sintering inhibition of Mo, the formation of a mixture of oxide phases in the oxidized state, and the cyclic formation of submicron Fe2Mo by reduction of MoO2 and Fe2Mo3O8. The microstructure of the foamed walls is examined throughout cycling to track changes in morphology and composition. These are correlated to volume and porosity changes in the lattice. A comparison is made to previously‐studied Fe‐Mo and Fe‐Ni freeze‐cast lamellar foams, and variations in performance are discussed in the context of the different architectures.

In situ studies on the industrial production process for molybdenum dioxide, MoO2

AbstractIn the industrial production of molybdenum the reduction of molybdenum trioxide with hydrogen is a two‐step process. In order to get a deeper understanding of the chemistry involved, in situ studies were performed on the thermal behavior of molybdenum trioxide and the first reduction step to molybdenum dioxide. MoO3 shows a very strong anisotropy for thermal expansion (linear expansion coefficients α in 10−4 K−1: 1.32(4) for a, 5.5(1) for b, −0.246(5) for c). Upon heating in air, preferred orientation in XRPD patterns decreases and the color changes from grey to yellow, probably caused by decreasing oxygen vacancy concentration. The reduction of MoO3 by hydrogen was investigated via in situ X‐ray powder diffraction for varying heating rate, hydrogen flow and potassium content. The formation of the intermediate γ‐Mo4O11 shows a higher rate for increasing potassium content; it does not play a role whether potassium is present in the starting material MoO3 or in MoO2 formed in the reaction. The high‐temperature modification of K2MoO4 was found to crystallize in the α‐K2SO4 type (P63/mmc, a=6.3463(2) Å, c= 8.1331(2) Å). Heating up MoO3 in vacuum (1 Pa) with the same T,t protocol as above yields MoO2 even without the presence of hydrogen.

Spectroscopic visualization of intermediate phases during CVD synthesis of MoS2

Chemical vapor deposition (CVD) is the broadly used technique in the synthesis of 2D materials for various electronic and optoelectronic devices. In this study, we aim to understand the precise phase transition of molybdenum trioxide (MoO3), the most commonly used transition metal precursor, under the influence of sulfur in a CVD process that ultimately leads to the growth of mono-to few-layer molybdenum disulfide (MoS2) with varying morphologies. For this, we employed a space-confined growth method and carried out a methodical spectroscopic analysis to understand the step-by-step reduction of MoO3 under reaction with sulfur vapors and its subsequent transformation to MoS2 on a SiO2 substrate. The sulfur-promoted reduction of MoO3 forming molybdenum dioxide (MoO2), followed by molybdenum oxysulfide (MoOS2) to MoOS2/MoS2 hybrid crystals, and finally monolayer (few layer) triangle (hexagonal) MoS2 crystals, is discussed. The results show that the nucleation presumably initiated by oxy-chalcogenide micro-particles that serve as heterogeneous monolayer nucleation sites. We validate our findings with optical and scanning electron microscope images, micro-Raman, photoluminescence (PL), and x-ray photoelectron spectroscopy (XPS) studies. Our findings can contribute to a better understanding of the growth processes of sequential sulfurization reactions.

Study on reduction of MoO2 by carbon diffusion to prepare molybdenum powder

In this study, MoO2 was reduced by an unmixed carbon reduction method. Effects of reduction temperature, reduction time and distance between carbon and MoO2 on the phase composition of reduction products were investigated. Results show that the phase change during the reduction process are MoO2→Mo2C→Mo with the increase of temperature. The reduced product is Mo2C when reduction temperature is 1000 °C and 1050 °C. Pure Mo can be obtained when the reduction temperature increases to 1100 °C. The reduced product changed from nearly spherical Mo particles with fine molybdenum dioxide particles attached on surface to polyhedron Mo particles with smooth surface when the distance between carbon and MoO2 decreases from 10 mm to 1 mm. The reaction showed a similar change with the changes of the reaction time. The difference is that there will be residual Mo2C in reduced product when reduction time was 1 h. This method of carbon reduction can provide low carbon concentration by controlling the carbon distance to avoid the existence of molybdenum carbide in the product of molybdenum preparation.

In situ powder X-ray diffraction during hydrogen reduction of MoO3 to MoO2

The hydrogen reduction of molybdenum trioxide to molybdenum dioxide is not yet fully understood as evident by continuous scientific interest. Especially the effect of the potassium content on the reduction process has not yet been considered. We prepared several samples of molybdenum trioxide containing varying amounts of potassium by addition of potassium molybdate (K2MoO4). In situ powder X-ray diffraction experiments were then conducted to study the hydrogen reduction of these samples. We especially focused on the influence of the alkali content and on gaining insight into the importance of the intermediary product γ-Mo4O11. During the reduction process, MoO2 is formed from the reduction of MoO3, which then reacts with the starting material to form γ-Mo4O11. With increasing potassium content, the reduction rate is decreased and the fractional content of γ-Mo4O11 built up during the reduction process is increased. As evident from bulk sample reduction, this results in a significant increase in the grain size visualized via scanning electron microscopy. Our investigations once again underline the importance of γ-Mo4O11 on the morphology of the resulting MoO2 powder.

Mo2N as a high-efficiency catalyst for transfer hydrogenation of nitrobenzene using stoichiometric hydrazine hydrate

A straightforward and commonly-used approach of direct pyrolysis for the preparation of molybdenum nitride (Mo2N) is conducted via reduction of MoO3 under ammonia atmosphere. The as-synthesized Mo2N catalyst demonstrated high activity for transfer hydrogenation of nitrobenzene, giving a 99% aniline yield at near-room temperature of 30 °C within only 50 min. More significantly, the amount of the hydrogen donor hydrazine monohydrate (N2H4·H2O) can be limited to stoichiometric molar ratio (-NO2: N2H4·H2O = 1: 1.5), preventing the possible generation of hazardous NH3, which is seldom reported in previous protocols. Through various characterizations and control experiments, conclusively, the remarkable catalytic performance of Mo2N catalyst should be credited to the large quantity of Lewis-acidic Mo sites. This research might provide a practical approach to aromatic amines production.

The Electrolytic Reduction of MoO3 in CaCl2-NaCl Molten Salt

Electro-deoxidation reduction technology is a very attractive method used to treat oxides in metallurgical industry. In this paper, cyclic voltammetry, square wave voltammetry and open circuit chronopotentiometry were applied to study the electrochemical behavior of MoO3 cathode in the CaCl2-NaCl melt at 873 K. Through the electrolytic reduction of MoO3 at different potentials (−1.15 V, −1.45 V and −1.75 V, vs Ag/AgCl) and XRD analysis, MoO3 was found to be transformed into several intermediate compounds (CaMoO4, MoO2 and Na1.4Mo2O4) and finally reduced to metallic Mo. When the electrolysis time was increased from 3 to 18.5 h and the employed potential was reduced from −1.75 V to −2.30 V (vs Ag/AgCl), the intermediate compounds could be completely reduced to metallic Mo, and the reduction ratio of MoO3 was calculated to be 93.7%.

In Situ X-ray Diffraction Studies on the Production Process of Molybdenum.

During the production of molybdenum, the first reduction step of molybdenum trioxide to molybdenum dioxide is crucial in directing important product properties like particle size and oxygen content. In this study, the influence of heating rate, hydrogen flow, and potassium content on the reduction of MoO3 has been investigated via in situ X-ray powder diffraction. For low heating rates, a molybdenum bronze HxMoO3 could be confirmed as an intermediate, while γ-Mo4O11 can only be observed at high heating rates. Molybdenum formation at temperatures as low as 873 K can be controlled via hydrogen flow. The potassium content of reactants has a direct influence on the amount of Mo4O11 formed during the reaction as well as rates of Mo4O11 and MoO2 formation.

A study of in situ reduction of MoO3 to MoO2 by X-ray Photoelectron Spectroscopy

Results from X-ray Photoelectron Spectroscopy (XPS) of molybdenum oxide samples are presented to elucidate how Mo (VI) oxide evolves to Mo (IV) oxide upon heating of a MoO3 sample. XPS data analysis techniques based on manipulation of spectra treated as vectors are shown and allow insights into intermediate phases of Mo oxide which suggest how the original oxide changes under the influence of heat but also supports an interpretation of as-received tetravalent Mo powders as multivalent materials. In particular, several new spectral components were observed and assigned to the Magnéli phase as well as MoO3 domains that have been modified by the presence of X-rays or temperature. These assignments comprise a new method of data processing where sample modification is used to inform XPS data interpretation and differ from previous work where the linear relationship of the binding energy of molybdenum oxides has been used.

Preparation and characterization of isotopically pure Mo targets for nuclear science measurements

Isotopically pure molybdenum targets are essential in various nuclear science measurements. For studies of cross sections of capture reactions on molybdenum, uniform targets with areal densities of hundreds of μg/cm2 are desired. Mechanical rolling of enriched metal pieces or vacuum sputtering methods used in previous studies to prepare molybdenum targets are challenging and involve labor- and time-intensive processes. Therefore, reliable and straightforward methods to produce molybdenum targets are crucial. We have developed a new double-step method to produce targets meeting these needs. The first step consists of evaporating isotopically enriched MoO3 onto gold backings. The second step involves the reduction of the oxide layer in a 5% H2-Ar environment to obtain molybdenum targets. X-ray fluorescence (XRF) spectroscopy, X-ray Diffraction (XRD), a scanning electron microscope/focused ion beam milling, and energy-dispersive X-ray spectroscopy (EDS) were used to characterize the targets. These methods show that the targets exhibit a uniform coverage of Mo over the gold backings. However, significant volume shrinkage occurring during the reduction of MoO3 creates a porous Mo layer where the gold diffuses due to the high processing temperatures, resulting in mixed Mo–Au layers.

Mechanism of Non-Contact Reduction of MoO3 to Prepare MoO2

Mechanism of Non-Contact Reduction of MoO3 to Prepare MoO2

Overturning CO2 Hydrogenation Selectivity with High Activity via Reaction-Induced Strong Metal-Support Interactions.

Encapsulation of metal nanoparticles by support-derived materials known as the classical strong metal-support interaction (SMSI) often happens upon thermal treatment of supported metal catalysts at high temperatures (≥500 °C) and consequently lowers the catalytic performance due to blockage of metal active sites. Here, we show that this SMSI state can be constructed in a Ru-MoO3 catalyst using CO2 hydrogenation reaction gas and at a low temperature of 250 °C, which favors the selective CO2 hydrogenation to CO. During the reaction, Ru nanoparticles facilitate reduction of MoO3 to generate active MoO3-x overlayers with oxygen vacancies, which migrate onto Ru nanoparticles' surface and form the encapsulated structure, that is, Ru@MoO3-x. The formed SMSI state changes 100% CH4 selectivity on fresh Ru particle surfaces to above 99.0% CO selectivity with excellent activity and long-term catalytic stability. The encapsulating oxide layers can be removed via O2 treatment, switching back completely to the methanation. This work suggests that the encapsulation of metal nanocatalysts can be dynamically generated in real reactions, which helps to gain the target products with high activity.

In Situ Powder X-Ray Diffraction During Hydrogen Reduction of Moo3 to Moo2

In Situ Powder X-Ray Diffraction During Hydrogen Reduction of Moo3 to Moo2

Cu-Mo nanocomposite preparation by combining solution combustion synthesis and self propagating high temperature synthesis

Cu-Mo composite powders were produced by combining solution combustion synthesis and self-propagating high-temperature synthesis. In the first stage a homogeneous nanocomposite powder comprising molybdenum dioxide and copper with average particle size of 50 nm was produced using ammonium heptamolybdate, copper nitrate as precursors and citric acid as a chelating reagent. At the second stage Cu-Mo nanocomposite preparation was performed by reduction of MoO2+Cu mixture with Mg + C combined reducer at a relatively moderate thermal conditions. The relationship between combustion peculiarities, phase composition and microstructural features were examined.

The Mechanism of Joint Reduction of MoO3 and CuO by Combined Mg/C Reducer at High Heating Rates

Understanding of the decisive role of non-isothermal treatment on the interaction mechanism and kinetics of the MoO3-CuO-Mg-C system is highly relevant for the elaboration of optimal conditions at obtaining Mo-Cu composite powder in the combustion processes. The reduction pathway of copper and molybdenum oxides with combined Mg + C reducing agents at high heating rates from 100 to 5200 K min−1 was delivered. In particular the sequence of the reactions in all the studied binary, ternary and quaternary systems contemporaneously demonstrating the effect of the heating rate on products’ phase composition and microstructure was elucidated. The combination of two highly exothermic and speedy reactions (MoO3 + 3Mg and CuO + Mg vs. MoO3 + CuO + 4Mg) led to a slow interaction with weak self-heating (dysynergistic effect) due to a change in the reaction mechanism. Furthermore, it has been shown that upon the simultaneous utilization of the Mg and C reducing agents, the process initiates exclusively with carbothermic reduction, and at relatively high temperatures it continues with magnesiothermic reaction. The effective activation energy values of the magnesiothermic stages of the studied reactions were determined by Kissinger isoconversional method.

On the Reduction of MoO3 to MoO2 : A Path to Control the Particle Size and Morphology.

During the reduction of molybdenum trioxide (MoO3) to metallic molybdenum, the first reduction step yielding molybdenum dioxide as an intermediary product is of crucial importance. In this study, we examined the impact of the parameters reduction temperature, water influx, and potassium content on the hydrogen reduction of this first reaction step. Beginning from the same starting material, the chemical vapor transport mechanism was utilized to yield the phase pure MoO2. Analyses including powder X‐ray diffraction, inductively coupled plasma‐mass spectrometry, scanning electron microscopy, and high performance optical microscopy were performed on the product phases. Modulations of the specific surface areas of molybdenum dioxide ranging from 2.28 to 0.41 m2/g were possible. Furthermore, a distinct shift from small plate‐like grains to cuboid‐like forms was achieved.

The high-performance MoO3−x/MXene cathodes for zinc-ion batteries based on oxygen vacancies and electrolyte engineering

Increasingly severe energy and environmental issues have prompted the development of high-quality energy storage. Aqueous zinc ion batteries (AZIBs) are one of the most attractive candidate devices because they have the advantages of safety and moderate energy density. MoO3 is considered as a promising cathode material for energy storage with huge potential windows due to its higher theoretical capacity and excellent electrochemical activity. However, the low conductivity and poor stability of MoO3 in aqueous seriously affect its wide application. To solve the problems, Ti3C2Tx MXenes with a two-dimensional (2D) layered structure and hydrophobic trifluoromethane sulfonate (OTf-) were introduced into the AZIBs. The MXenes not only causes partial reduction of MoO3 to form oxygen vacancies but also reduces the hydrophilicity of MoO3. The OTf- in zinc trifluoromethane sulfonate (Zn(OTf)2) electrolyte is conducive to prevent cathode from dissolving. As expected, a paper of MoO3−x/MXene delivers exceptional high capacity of 369.8 mAh g−1 at 0.20 A g−1 and an outstanding cycling life of 46.7% capacity retention after 1600 cycles. A zinc ion micro-battery (ZIMB) employing the MoO3−x/MXene paper as cathode demonstrates superior performance and a high capacity. This study paves a new strategy for the design of cathode materials with high capacity, stability, and commercialization of AZIBs.

Metallothermic Reduction of MoO3 on Combustion Synthesis of Molybdenum Silicides/MgAl2O4 Composites

Combustion synthesis involving metallothermic reduction of MoO3 by dual reductants, Mg and Al, to enhance the reaction exothermicity was applied for the in situ production of Mo3Si–, Mo5Si3− and MoSi2–MgAl2O4 composites with a broad compositional range. Reduction of MoO3 by Mg and Al is highly exothermic and produces MgO and Al2O3 as precursors of MgAl2O4. Molybdenum silicides are synthesized from the reactions of Si with both reduced and elemental Mo. Experimental evidence indicated that the reaction proceeded as self-propagating high-temperature synthesis (SHS) and the increase in silicide content weakened the exothermicity of the overall reaction, and therefore, lowered combustion front temperature and velocity. The XRD analysis indicated that Mo3Si–, Mo5Si3– and MoSi2–MgAl2O4 composites were well produced with only trivial amounts of secondary silicides. Based on SEM and EDS examinations, the morphology of synthesized composites exhibited dense and connecting MgAl2O4 crystals and micro-sized silicide particles, which were distributed over or embedded in the large MgAl2O4 crystals.