Reduction Of MoO3 Research Articles

Mechanism and kinetic study of hydrogen reduction of ultra-fine spherical MoO3 to MoO2

The mechanism and kinetics of hydrogen reduction of ultra-fine spherical MoO3 to MoO2 have been investigated in the present work. The results show that the reduction of MoO3 to MoO2 obeys the two-step reduction mechanism with the generation of intermediate product Mo4O11. The final product MoO2 always keeps the same morphology as Mo4O11. The experimental data can be well described by using the dual interface reaction model. It was found that the rate controlling steps for the first (from MoO3 to Mo4O11) and second reactions (from Mo4O11 to MoO2) were interface chemical reaction and diffusion, respectively, with the activation energies extracted to be 122kJ/mol and 114kJ/mol. When the reaction extent (defined as the ratio of weight loss of MoO3 at time t to the theoretical maximum weight loss from MoO3 to MoO2 due to the removal of oxygen during the reduction process) is in the range of 0 to 0.7, both the reduction of MoO3 to Mo4O11 and Mo4O11 to MoO2 occurred simultaneously; when the reaction extent is in the range of 0.7 to 1, only the reduction of Mo4O11 to MoO2 occurred in the reduction process.

On the formation of Mo2C nanocrystals by a novel system through microwave assisted combustion synthesis

This research is devoted to microwave assisted combustion synthesis of Mo2C nanoparticles. The ternary system of MoO3–Zn–C was used as a novel approach for the in-situ synthesis of Mo2C in which the zincothermic reduction of MoO3 was responsible for the combustion to take place. Results showed that the formation of Mo2C was assisted by the zincothermic reaction, although further microwave heating up to 6min was necessary to complete the reaction. The effects of the microwave heating and mechanical activation on the reaction progress were investigated. X-ray powder diffraction was used to examine the synthesis progress. Final products (Mo2C and ZnO) were successfully separated from each other and the synthesized carbide was characterized by transmission electron microscopy (TEM), showing the formation of Mo2C hexagonal nanocrystals during combustion process.

The effect of preliminary mechanical activation on the zinc loss control in combustive reduction of MoO3 by Zn

Due to the high temperature at combustion moment, the combustive reduction of MoO3 by Zn is normally accompanied with a great deal of zinc vaporization and the formation of undesired compound, Zn2Mo3O8. Experimental results showed that there was about 23wt.% Zn loss at the combustion moment as a result of temperature rising. In the present research, preliminary mechanical activation was employed to control Zn loss during the combustion. It was shown that the decrease in zinc loss is associated with the intensity of structural disorder generated during mechanical milling. Zinc loss was calculated to be about 4wt.% as the pre-milling time increased up to 30min. X-ray diffraction, electron microscopic images (SEM and TEM), EDS mapping, and differential scanning calorimetry were used in this research to study the synthesis process. TEM results revealed that synthesized Mo particles were nanosized and under 50nm on average.

Facile synthesis of high surface area molybdenum nitride and carbide

The synthesis of high surface area γ-Mo2N and α-Mo2C is reported (116 and 120m2/g) without the temperature programmed reduction of MoO3. γ-Mo2N was prepared in an NH3-free synthesis using forming gas (7at% H2, N2-balance) as the reactive atmosphere. Three precursors were studied ((NH4)6Mo7O24·4H2O, (NH4)2Mg(MoO4)2, and MgMoO4) along with the sacrificial support method (SSM) as a means of reducing the particle size of Mo2N and Mo2C. In situ X-ray diffraction (XRD) studies were carried out to identify reaction intermediates, the temperature at which various intermediates form, and the average domain size of the Mo2N products. Materials were synthesized in bulk and further characterized by XRD, HRTEM, XPS, and BET.

Reduction of MoO<sub>2</sub> to Metallic Molybdenum via Carburization and Thermal Process

A metallic molybdenum is formed by a two-stage process that was carburized MoO2 with carbon monoxide followed by heat treatment. XRD techniques were employed to identify the phase’s changes in the sample. It was found that by the present of carbon monoxide during carburization process until 900 °C, Mo2C is formed first, compared to metallic molybdenum, due to the high thermodynamic barrier for formation of metallic molybdenum. In the second stage, Mo2C further reacts with the remaining MoO2 at 900 °C to produce metallic molybdenum.

A mechanistic study on the production of nanosized Mo in microwave assisted combustive reduction of MoO3 by Zn

The combustion synthesis of elemental Mo nanoparticles by zincothermic reduction of MoO3 was investigated in this research. It was determined that MoO3 reduces to Mo by means of two sub-reactions. The results showed that MoO3 reduces initially to MoO2 and subsequently dioxide phase reduces to elemental Mo. Due to the formation of MoO2 during the combustion moment, a considerable amount of unwanted Zn2Mo3O8 (2ZnO·3MoO2) phase was synthesized. Since the presence of this phase would prevent the reduction to be completed, hence this article was devoted to introduce some ways by which the zincothermic reduction of MoO3 can be completed. In one approach, the results indicated that by increasing the microwave heating duration after combustion moment, the unwanted Zn2Mo3O8 compound reduced successfully to Mo. In the other approach, the use of over-stoichiometric amounts of Zn led to prevent the formation of the undesired phase. In this research, X-ray diffraction technique (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to characterize the products of the reactions.

In situ synthesis and high adsorption performance of MoO₂/Mo₄O₁₁ and MoO₂/MoS₂ composite nanorods by reduction of MoO₃.

MoO2/Mo4O11 and MoO2/MoS2 composite nanorods were successfully synthesized by reducing MoO3 nanorods. The adsorption performance of samples was evaluated by using Rhodamine B (RhB) and metal ions in aqueous solutions as the targets. The results indicated that the obtained MoO2/Mo4O11 and MoO2/MoS2 composite nanorods can present excellent adsorption performance. The BET surface areas of samples increased with increasing the MoO3 : S mass ratio. The adsorption kinetics for the RhB and Cu(2+) ions on the composite nanorods were well fitted to a pseudo-second order model.

Root-cause failure analysis of photocurrent loss in polythiophene:fullerene-based inverted solar cells.

Metal oxide transport layers have played a crucial role in recent progress in organic photovoltaic (OPV) device stability. Here, we measure the stability of inverted and encapsulated polythiophene:fullerene cells with MoO3/Ag/Al composite anode in operational conditions combining solar radiation and 65 °C. Performance loss of over 50% in the first 100 h of the aging is dominated by a drop in the short-circuit current (Jsc). We reveal a concurrent loss in reflectance from 85% to 50% above 650 nm, which is below the optical gap of the used photoactive materials, hence, excluding any major degradation in the bulk of this layer. Correlating the responses of aged devices to a series of test structures comprised of ITO/ZnO cathode, MoO3/Ag, and MoO3/Ag/Al anodes and their combinations with the active layer allowed us to identify that the presence of Al causes the reduced reflectance in these devices, independent of the presence of the active layer. Systematic single-stress aging on the test structures further indicates that elevated heat is the cause of the reflectance loss. Cross-section transmission electron microscopy coupled with elemental analysis revealed the unsuspected role of Al; notably, it diffuses through the entire 150 nm thick Ag layer and accumulates at the MoO3/Ag interface. Moreover, XRD analysis of the aged MoO3/Ag/Al anode indicates the formation of Ag2Al alloy. Depth profiling with X-ray photoelectron spectroscopy advanced our understanding by confirming the formation of Ag-Al intermetallic alloy and the presence of oxidized Al only at the MoO3/Ag interface suggesting a concomitant reduction of MoO3 to most probably MoO2. This latter compound is less reflective than MoO3, which can explain the reduced reflectance in aged devices as proven by optical simulations. On the basis of these results, we could estimate that 20% of the loss in Jsc is ascribed to reduction of MoO3 triggered by its direct contact with Al.

Tailoring acid–metal functions in molybdenum oxides: Catalytic and XPS-UPS, ISS characterization study

Formation of acid, metal and metal–acid (bifunctional) functions in titania-supported molybdena following controlled partial reduction of MoO3 using hydrogen at different temperatures are monitored by a combination of surface XPS-UPS, ISS techniques. Addition of controlled amount of sodium or potassium alkali metals to the bifunctional MoO2−x(OH)y/TiO2 (MoTi) enabled to neutralize the Brønsted acid Mo-OH functions. Sodium or potassium molybdenum bronze metallic function is formed following the addition of the alkali metal to the Mo salt following its calcination at 773K. The catalytic functions of these MoTi and Na, KMoTi systems were evaluated for the dehydration/hydrogenation, oxidation reactions of isopropanol to acetone as well as the ring shortening of cyclohexane to methylcyclopentane MCP and its oxidative dehydrogenation to benzene.

New method for preparation of low oxygen Mo powder by reduction of MoO3 using Ca

A new method for the reduction of MoO3 using Ca as a reducing agent was developed. A low oxygen Mo powder can thus be prepared by two-step reduction process: the first step reduction involves a contact type method while the second step reduction uses a non-contact type method involving Ca vapour followed by additional deoxidation. The oxygen concentration in the resultant Mo powder was ∼2800 ppm, which was much lower than that of commercial Mo powder (>4400 ppm).

Phase Transitions and Morphology Evolutions during Hydrogen Reduction of MoO3 to MoO2

Abstract In the present work, studies of phase transitions and morphology evolutions during hydrogen reduction of MoO3 to MoO2 were carried out in the temperature range from 678 to 829 K. It was found that no intermediate phase was formed and MoO2 was the unique detectable product during the reduction at 678 K. While above 713 K, an intermediate phase of Mo4O11 was formed. The formation of Mo4O11 was found to obey the consecutive mechanism under the experimental condition. Regarding the morphology evolution of MoO2 during reduction, it was found that in the temperature range from 713 to 733 K, MoO3 was firstly reduced to large spherical or oval Mo4O11 grains, then to small spherical or oval MoO2 ones. From 793 to 829 K, most of MoO3 grains were firstly reduced to large plateshaped Mo4O11 grains, subsequently to small plateshaped MoO2 ones.

Carbothermic Reduction of MoO3 for Direct Alloying Process

The thermodynamics of Mo-O-C and Ca-Mo-O-C systems was studied in order to understand the carbothermic reduction of molybdenum trioxide, and kinetic studies were also carried out by means of thermogravimetric analysis under argon atmosphere with a heating rate of 10 °C/min. Subsequently, reaction products at various temperatures were identified by X-ray diffraction (XRD) and the results confirmed the previous thermodynamics analysis. Meanwhile, it was found that intermediate products MoO2 and CaMoO4 appeared in the process of carbothermic reduction of MoO3 with or without CaO, which were subsequently reduced to Mo or molybdenum carbide. An experimentally determined reaction mechanism was proposed and discussed. The reduction reaction of MoO3 with carbon could be divided into two stages. The first stage includes the direct reaction between MoO3 and carbon and the carbon gasification reaction. The second stage is the gas-solid reaction between CO and MoO2, and the diffusion of gases through the surface of MoO2 determines the overall reaction rate. The activation energies of the mixtures with or without CaO were estimated to be 56.6 and 52.9 kJ/mol, respectively.

Effect of processing parameters on the mechanochemical synthesis of nanocrystalline molybdenum carbide

Nanocrystalline molybdenum carbide (Mo2C) was synthesized from commercially pure MoO3 and graphite using high energy planetary ball milling and subsequent heat treatment. X-ray diffraction (XRD), differential thermal analysis/thermogravimetry (DTA/TG) and field emission scanning electron microscopy (FE-SEM) techniques were employed to evaluate powder particle characteristics. XRD results showed that during carbothermic reduction of MoO3, metallic molybdenum is formed as an intermediate phase before the carbide formation. Phase composition of the 20h milled sample with 90% extra graphite after heat treatment at 1000°C consists of nano-crystalline Mo2C with a mean crystallite size of 54nm.

Effect of molybdenum carbide intermediate layers on thermal properties of copper–diamond composites

Abstract Molybdenum carbide (Mo 2 C) coated diamond particles were prepared by molten salts method and the copper–diamond composites were obtained by vacuum pressure infiltration of Mo 2 C-coated diamond particles with pure copper. The formation mechanism of the Mo 2 C layers was investigated, and the microstructure, thermal conductivity and thermal expansion behavior of the obtained copper–diamond composites were studied. The coated layers were formed by using a reaction medium of MoO 3 in mixed molten salts, and the formation mechanism of the Mo 2 C layers on diamond particles was investigated. The result indicates that the formation of Mo 2 C layers occurred in two steps, that is, the reduction of MoO 3 to MoO 2 and the reduction of MoO 2 to Mo 2 C. The wettability between diamond particles and copper was effectively improved, and the relative density of the copper–diamond composites achieved 99.5%. The thermal conductivity reached 596 W m −1 K −1 , and the coefficient of thermal expansion was 7.15 × 10 −6 K −1 of the composites with 60 vol.% Mo 2 C-coated diamond. This composite is expected to be suitable for electronic packaging applications.

Kinetics and mechanism of hydrogen reduction of MoO3 to MoO2

Kinetics and mechanism of the hydrogen reduction of MoO3 to MoO2 were studied in this work. The experimental findings confirmed that the reduction of MoO3 to MoO2 was a consecutive reaction with the intermediate product Mo4O11. The dual reactions (MoO3→Mo4O11, Mo4O11→MoO2) occurred simultaneously, and the rate of the first reaction was higher than that of the second one. The reduction kinetics of MoO3 was analyzed according to a two-interface model, the predicted curves of which agreed well with the experimental results. Using this model, the rate controlling steps and the corresponding activation energies were obtained.

Assistance of partially reduced MoO3 interlayer to hole-injection at iron phthalocyanine/ITO interface evidenced by photoemission study

Molybdenum trioxide (MoO3) has been widely used as an interlayer between electrode and functional organic layer inducing significant improvement in the performance of organic electronic and optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). In this work, the electronic structures and energy level alignment at FePc/MoO3 (3nm)/ITO and FePc/MoOx (3nm)/ITO (x<3) interfaces have been comparatively investigated in situ by means of photoemission spectroscopy (PES) in an attempt to understand the influence of MoOx layer on the interfacial hole injection property. It is found that the electron injection barrier is only 0.45eV for electron extraction from the highest occupied molecular orbital (HOMO) of FePc to the conduction band of MoOx. Simultaneously, two gap states at binding energy of 1.15eV and 2.30eV present in 3nm MoOx, which extend from the valence band maximum of MoOx to the Fermi level. These gap states are suggested to originate from the partial occupation of Mo 4d states due to the reduction of MoO3 during the annealing process. Both lowered electron extraction barrier and presence of interfacial gap states appear to assist the hole injection from ITO anode into FePc by the insertion of MoOx layer.

Catalytic Active Sites in Molybdenum Based Catalysts

In situ metal, acid and metal-acid (bifunctional) catalytic active functions were prepared following partial reduction by hydrogen of MoO3 deposited on TiO2 at temperatures between 623 K and 673 K. The bifunctional structure is obtained following the reduction of MoO3 to MoO2. The metallic properties of MoO2 are attributed to the delocalized p electrons above the Mo atoms place along the C-axis of the deformed rutile structure of this phase and observed as a density of states at the Fermi level. Hydrogen dissociation by this metallic function and bonding of the produced H atoms to surface oxygen atoms results in the formation of Bronsted acid Mo-OH function(s). Accordingly, a bifunctional (metal-acid) MoO2-x(OH)y structure is formed on the TiO2 support. The bifunctional properties enabled to perform isomerization reactions of light naphtha hydrocarbons into branched species of higher octane number. This catalyst is proposed as a possible replacement of the commercially used Pt deposited on chlorinated alumina catalysts in which toxic chlorine is employed and benzene is produced as a byproduct of n-hexane isomerization. The acid function in this bifunctional Mo system is quenched following the addition of controlled amount of sodium. The presence of only the metallic function in this modified NaMoTi system is monitored via the hydrogenation of olefins and enabled to define the bifunctional mechanism of the hydrocarbon isomerization process performed by MoO2-x(OH)y structure.

Metal/Metal‐Oxide Interfaces: How Metal Contacts Affect the Work Function and Band Structure of MoO3

AbstractWhen transition metal oxides are used in practical applications, such as organic electronics or heterogeneous catalysis, they often must be in contact with a metal. Metal contacts can affect an oxide's chemical and electronic properties within the first few nanometers of the contact, resulting in changes to an oxide's chemical reactivity, conductivity, and energy‐level alignment properties. These effects can alter an oxide's ability to perform its intended function. Thus, the choice of contacting metal becomes an important design consideration when tailoring the properties of transition‐metal oxide thin films or nanoparticles. Here, metal/metal‐oxide interfaces involving a widely used oxide in organic electronics, MoO3, are examined. It is demonstrated that metal contacts tend to reduce the Mo6+ cation to lower oxidation states and, consequently, alter MoO3’s valence electronic structure and work function when the oxide layer is very thin (less than 10 nm). MoO3 becomes semimetallic and has a lower work function near metal contacts. The observed behavior is attributed to two causes: 1) charge transfer from the metal Fermi level into MoO3’s low‐lying conduction band and 2) an oxidation‐reduction reaction between the metal and MoO3 that results in oxidation of the metal and reduction of MoO3. These results illustrate how interfaces are important to an oxide's ability to provide energy‐level alignment.

Rate control mechanism for the hydrogen reduction of MoO3 to MoO2

Rate control mechanism for the hydrogen reduction of MoO3 to MoO2

Wafer-scale MoS2 thin layers prepared by MoO3 sulfurization

Atomically thin molybdenum disulfide (MoS(2)) layers have attracted great interest due to their direct-gap property and potential applications in optoelectronics and energy harvesting. Meanwhile, they are extremely bendable, promising for applications in flexible electronics. However, the synthetic approach to obtain large-area MoS(2) atomic thin layers is still lacking. Here we report that wafer-scale MoS(2) thin layers can be obtained using MoO(3) thin films as a starting material followed by a two-step thermal process, reduction of MoO(3) at 500 °C in hydrogen and sulfurization at 1000 °C in the presence of sulfur. Spectroscopic, optical and electrical characterizations reveal that these films are polycrystalline and with semiconductor properties. The obtained MoS(2) films are uniform in thickness and easily transferable to arbitrary substrates, which make such films suitable for flexible electronics or optoelectronics.