Mo-based Compounds Research Articles

Mo-based compounds coating to stabilize Mn-based layered oxide cathode material towards high-performance Na-ion batteries with ultrahigh cycling stability

Layered oxide cathode materials have good structural stability and high capacity, making them ideal cathode materials of Na-ion batteries (SIBs). Nonetheless, the irreversible phase transitions and metal dissolution often result in poor cycling stability. Here, a simple strategy was developed to Na2MoO4 and MoO3 coated Na0.67Fe0.15Cu0.1Mn0.65Ni0.1O2 (NFCMN-M) as cathode material of SIBs with ultrahigh cycling stability caused by the enhanced kinetic. The formed Na2MoO4 layer on the surface of Na0.67Fe0.15Cu0.1Mn0.65Ni0.1O2 (NFCMN) particles supplies a good conductive connection, and inhibits side effects between cathode material and electrolyte. Thanks to the unique component, the as-prepared NFCMN-M shows a good rate performance with a reversible discharge capacity of 132.4, 112.6, 102.3, 87.1 and 79.3 mAh g−1 at 0.1, 0.5, 1, 3 and 5C, respectively. The NFCMN-M also delivers an initial capacity of 73.7 mAh g−1 at 10C and an ultrahigh cycling stability with a capacity retention rate of about 80 % after 1000 cycles. The result confirms that the Na2MoO4 modification is a direct and effective strategy to improve the capacity and enhance cycling stability of NFCMN at high charge-discharge rates.

Structural, optical and electrical properties of NiMO4 (M=W and Mo)

The NiMO4 (M=Mo and W) samples have been synthesized using the solid-state reaction method at high temperatures. The structural, optical, and electrical transport properties were investigated. X-ray diffraction data refined via the Rietveld method showed the monoclinic system for these compounds. The UV/Vis optical spectra indicate that the direct gap energy values are 2.72 eV for NiMoO4 and 2.81 eV for NiWO4 compounds which can be promising candidates for semiconductor applications. These materials were characterized by an impedance spectroscopy technique measured in the 1 – 106 Hz frequency and 398 – 533 K temperature range. The AC conductivity was analyzed using the Jonscher power law, which showed that the conduction mechanisms are well described by the non-overlapping small polaron-tunneling model (NSPT) in phase I and the correlated barrier-hopping model (CBH) in phases II for the Mo-based compound. Moreover, the overlapping-large polaron tunneling (OLPT) model for the NiWO4 compound. The conduction mechanism was interpreted with the help of Elliot's theory, and the Elliot's parameters were determined. The comparative study of the two titled compounds shows better properties for the Mo-based compounds.

Ru-doped MoO2/Mo2C coupled with N-doped carbon as an efficient pH-universal electrocatalyst for hydrogen generation

The preparation of stable, efficient and pH-universal hydrogen evolution reaction (HER) electrocatalysts is urgent and significant for large-scale hydrogen production. Herein, a simple and efficient method was developed to fabricate Ru-doped MoO2/Mo2C onto N-doped carbon nanoparticles (Ru-MoO2/Mo2C/NC). The formed smaller nanoparticles by Ru doping can provide abundant accessible active sites and interfaces. Moreover, the synergistic effects of Ru and Mo-based compounds also played a key role in promoting the catalytic kinetics. The as-prepared Ru-MoO2/Mo2C/NC exhibited low overpotentials of 38, 61, 67 and 75 mV in alkaline, neutral, acidic electrolyte solutions and alkaline seawater at 10 mA cm−2, respectively. This work proposes a valid method for the design of efficient electrocatalysts through precise component regulation.

Impact of annealing on charge storage capability of thermally evaporated molybdenum oxide thin films

Rapid technological advancements in recent years have necessitated the creation of energy-related devices. Due to their unique properties, including an exceptional cycling life, safe operation, low processing cost, and a higher power density than batteries, supercapacitors (SCs) have been identified as one of the most promising candidates to meet the demands of human sustainability. To increase the energy density of SCs, several advanced electrode materials and cell designs have been researched during the past few years. Utilizing the Faradaic charge storage process of transition metal cations, transition metal compounds have recently received attention as prospective electrode materials for SCs with high energy densities. In this work, the structure, morphology and electrochemical properties of molybdenum oxide (MoO3) films deposited via thermal evaporation technique and annealed at various temperatures were systematically investigated in order to examine the potential use of MoO3 for supercapacitors. Electrochemical analysis confirmed the pseudocapacitance characteristics of the synthesized films. The annealing temperature affects the oxidation and reduction observed in the cyclic voltammetry (CV) plots. Areal capacitance of thin films annealed at 150°C was found to be maximum and this could be attributed to the formation of hollow tube-like nanostructures which provided more active sites, than films annealed at higher temperatures. This also influences the charge storage ability of the synthesized films. It would be logical to assume that additional research in this area will result in more interesting discoveries and, eventually, the vi-ability of those promising Mo-based compounds in high-tech energy storage systems.

Hierarchical α-MoC1-x@N-doped carbon nanospheres as counter electrodes for dye-sensitized solar cells

Molybdenum-based carbides, as one of the representatives of transition metal compounds, are a promising alternative to the expensive and resource constrained Pt-based counter electrode (CE) in dye-sensitized solar cells (DSSCs). Herein, a series of hierarchical α-MoC1-x@N-doped carbon (MDNC) composites are transformed from the step-by-step pyrolysis of uniformly Mo-based compounds and dopamine precursors at the temperatures of 650, 750, 850, and 950 °C under N2 atmosphere, respectively. The MDNC precursors were synthesized by a simple one-step self-polymerization in a water-in-oil system. And then, the obtained MDNC nanocrystalline composites can accelerate electron conduction and improve the electrocatalytic property of the tri-iodide reduction reaction (IRR) owing to the C carrier obtained through the pyrolysis process and the N-doped of α-MoC1-x by the carbonization of dopamine and the high temperature nitrogen atmosphere. In addition, the distinct hierarchical structure of MDNC with well-developed porosity and high specific surface area can expose numerous active catalytic sites. The DSSCs using the MDNC series as CEs for I3−/I− exhibit power conversion efficiencies (PCE) of 5.99 %, 6.35 %, 6.57 % and 5.57 %, respectively, which were versus 5.83 % of Pt CE.

Promoting polysulfide redox kinetics by tuning the non-metallic p-band of Mo-based compounds

This work studies the kinetic behaviors of Li–S chemistry on Mo-based compounds (MCNs-MoXn, X = O, S, N, P), and it is found that the Li–S battery using the MoP-modified separator exhibits superior rate and long cycle performance.

Chemical Information in the L3 X-ray Absorption Spectra of Molybdenum Compounds by High-Energy-Resolution Detection and Density Functional Theory.

X-ray spectroscopy using high-energy-resolution fluorescence detection (HERFD) has critically increased the information content in X-ray spectra. We extend this technique to the tender X-ray range and present a study at the L3-edge of molybdenum. We show how information on the oxidation state, phase composition, and local environment in molybdenum-based compounds can be obtained by analyzing the HERFD L3 X-ray absorption near-edge structure (XANES). We demonstrate that the chemical shift of the L3-edge HERFD spectra follows a parabolic dependence on the oxidation state and show that a qualitative analysis of high-resolution spectra can help to estimate parameters such as distortion of a ligand environment and radial order of atoms around the absorber. In certain cases, the spectra allow disentangling the contributions from bond lengths and angles to the distortion of the ligand polyhedron. Comparison of the high-resolution spectra with theoretical simulations shows that the single-electron approximation is able to reproduce the spectral shape. The results of this work may be useful in every branch of physics, inorganic and organometallic chemistry, catalysis, materials science, biochemistry, and mineralogy where observed changes in performance or chemical properties of Mo-based compounds, accompanied by small changes in spectral shape, are to be related to the details of electronic structure and local atomic environment.

Unlocking the catalytic activities of 2H-phase Mo-based compounds via topological conversion reaction

Although 2H molybdenum disulfide (MoS2) layers are highly desirable for hydrogen evolution reaction (HER) owing to their high chemical stabilities and low cost, the inert basal plane and semiconducting nature severely hinder their practical applications. Here, the catalytic activities of 2H phase molybdenum-based compounds are unlocked via topological conversion reaction from Mo2GeC MAX phase to accordion-like molybdenum phosphosulphide. During the conversion reaction, phosphorus atoms can be implanted into the sandwich-like S-Mo-S planes and gradually substitute sulfur atoms, which are beneficial to optimize the electronic configuration and facilitate to absorb water molecules, unlocking the inert basal planes of 2H phase molybdenum-based compounds. The accordion-like molybdenum phosphosulphide exhibits high hydrogen production rates up to 288 mmol g−1 h−1 cm−2 at a high loading of 3 mg cm−2 and ultralong durability up to 35,000 cycles, satisfying the practical application for HER.

Two-dimensional Mo-based compounds for the Li-O2 batteries: Catalytic performance and electronic structure studies

Rechargeable Li-O2 batteries have captured increasing attention owing to their ultra-high theoretical specific energy. However, this promising system is confronted with the large overpotential, limited discharge capacity and low cyclic life. Herein, three two-dimensional (2D) molybdenum-based compounds of MoN, MoO3 and MoS2 are used as cathode catalysts in Li-O2 batteries. The catalytic performance and electronic structures of these catalysts are compared comprehensively. Electrochemical test results reveal the superior battery performance of MoN cathodes, which deliver the highest specific discharge capacity of approximately 7400 mAh g−1 and the lowest discharge/charge overpotential of 0.19/0.72 V. Density functional theory (DFT) calculations demonstrate the metallic property of MoN whereas MoO3 and MoS2 are poor conductive. Besides, interface properties between MoN and Li2O2 products as well as reaction pathways of the Li-O2 battery with MoN cathode are also investigated detailedly by DFT calculations, explaining the excellent catalytic properties of MoN at the atomic level. The present work provides intrinsic insights into designing high-performance cathode catalysts for Li-O2 batteries with fine-tuned morphology and electronic characteristics.

Facile fabrication of molybdenum compounds (Mo2C, MoP and MoS2) nanoclusters supported on N-doped reduced graphene oxide for highly efficient hydrogen evolution reaction over broad pH range

• Develop new molybdenum compounds (Mo 2 C, MoP and MoS 2 ) nanoclusters supported on N-doped reduced graphene oxide. • Prepared catalysts exhibit excellent and durable electrocatalytic performance over a broad pH range. • Potential applications of ordered self-assembled composites for water splitting to produce hydrogen. Electrocatalytic water splitting for hydrogen production is highly desirable to replace oil energy. The catalytic performance is closely related to the conductivity, active sites and reaction Gibbs free energy of the catalyst. A general guideline for improving catalytic performance is to obtain porous carbon-based materials with a high surface area, plentiful defects and metal compound loading. Here, defect-rich nitrogen-doped reduced graphene oxide (RGO) with molybdenum (Mo)-based compound loading was designed through a two-step method. The peroxide-assisted step under low-temperature carbonization with an air atmosphere promotes the formation of defects, and the hydrothermal process improves the crosslinking degree to form a porous structure at a high carbonization temperature. The obtained catalysts exhibit excellent and durable electrocatalytic performance over a broad pH range. In addition, the transmission electron microscopy (TEM) and electron paramagnetic resonance (EPR) results clearly reveal the presence of defects. The theoretical analysis demonstrates that the RGO and Mo-based compounds have an efficient synergetic effect on the catalytic activity. This work provides clues for the development of new catalysts for water splitting to produce hydrogen.

Molybdenum recovery from spent Mo-based dispersed catalyst accumulated in heavy oil steam cracking coke

Recovery of molybdenum has been studied from coke formed as a by-product of catalytic steam cracking of heavy oil containing a Mo-based nanodispersed unsupported catalyst, and from mixtures of industrial coke with Mo-based compounds – as model systems, via combustion in the fluidized bed of deep oxidation catalyst and the fixed bed of coke (without a catalyst). Quantitative carbon removal was found to be impossible even for several cycles of combustion in fluidized catalyst bed, most probably due to a drop of size of the coke particles to those of too small in order to have a sufficient contact time for the complete burning off. Non-catalytic combustion in the fixed bed mode showed best efficacy resulting in complete carbon removal without any Mo losses in the range of 450–750 °C.

Advances in Catalyst Design for the Conversion of Methane to Aromatics: A Critical Review

The catalytic methane aromatization (CMA) via the non-oxidative approach is increasingly becoming an interesting process for natural gas, shale gas and biogas valorization. However, the design of catalytic materials that could produce very high methane conversion and aromatics selectivity at affordable stability is still difficult. We have carefully documented herein a comprehensive literature survey on the advances in different catalysts designed, characterized and evaluated for the CMA reaction in the recent times. Details on metal-modified zeolite systems including the active sites identification, role of different metals and their loadings, reaction mechanisms and influence of synthetic and catalytic parameters were substantially discussed. The scope was extended to include zeotypes, oxides, un-modified Mo-based compounds and associated catalysts recently evaluated. There are indications that the powdered GaN material is currently been considered as a CMA catalyst. Therefore, relevant studies were carefully surveyed and discussed. In all cases, parameters varied for ensuring conversion and selectivity control were simultaneously highlighted. The paper identified new research paths based on the areas partly explored for further investigations.

Valley-contrasting optics of interlayer excitons in Mo- and W-based bulk transition metal dichalcogenides.

Recently, spatially indirect ("interlayer") excitons have been discovered in bulk 2H-MoTe2. They are theoretically predicted to exist in other Mo-based transition metal dichalcogenides (TMDCs) and are expected to be present in W-based TMDCs as well. We investigate interlayer excitons (XIL) in bulk 2H-MoSe2 and 2H-WSe2 using valley-resolved magneto-reflectance spectroscopy under high magnetic fields of up to 29 T combined with ab initio GW-BSE calculations. In the experiments, we observe interlayer excitons in MoSe2, while their signature is surprisingly absent in WSe2. In the calculations, we find that interlayer excitons exist in both Mo- and W-based TMDCs. However, their energetic positions and their oscillator strengths are remarkably different. In Mo-based compounds, the interlayer exciton resonance XIL is clearly separated from the intralayer exciton X1sA and has a high amplitude. In contrast, in W-based compounds, XIL is close in energy to the intralayer A exciton X1sA and possesses a small oscillator strength, which explains its absence in the experimental data of WSe2. Our combined experimental and theoretical observations demonstrate that interlayer excitons can gain substantial oscillator strength by mixing with intralayer states and hence pave the way for exploring interlayer exciton physics in Mo-based bulk transition metal dichalcogenides.

Interfacial Interactions as an Electrochemical Tool To Understand Mo-Based Catalysts for the Hydrogen Evolution Reaction

Crystalline and amorphous transition-metal chalcogenides such as MoS2 are currently recognized as state of the art non-precious transition metal catalysts for the hydrogen evolution reaction (HER). Nevertheless, despite numerous studies dedicated to their electrocatalytic activities, the exact nature of the active sites and their interaction with interfacial water remain largely elusive. In this work, amorphous and crystalline MoS2 catalysts were prepared by electrodeposition and chemical exfoliation, respectively, and compared with other Mo-based compounds. Herein, we show that all of these compounds exhibit two reduction mechanisms in low proton concentration: proton reduction occurs at low overpotential followed by water reduction at higher overpotential. We show that both the chemical composition and the structure of the catalyst influence the activity of the proton reduction but that none of those materials efficiently catalyze water reduction. Finally, we demonstrate by using different cations (Li+,...

A mechanism for the selective epimerization of the glucose mannose pair by Mo-based compounds: towards catalyst optimization

The reducibility of Mo centers of H3PMo12O40 allows the selective C2 epimerization of glucose/mannose through a Mo-assisted 1,2 C-shift.

Electronic and magnetic properties of Sr2MoBO6 (B=W, RE, Os): Investigation of possible half metal

The magnetic ordering temperatures of Sr2CrBO6 (B=W, Re, Os) are the top three in the class of double perovskites so far, whereas among them only Sr2CrWO6 is a half metal. In this study, by substituting Cr with Mo, Sr2MoBO6 is investigated by using the density functional theory. The calculated results indicate that all the three Mo-based compounds exhibit the half metallic nature, in particular Sr2MoOsO6 is a compensated half metal. On the other hand, Sr2MoBO6 is estimated to have at least a comparable magnetic ordering temperature with that of Sr2CrOsO6 (experimental value of 725K). Therefore, we expect that Sr2MoBO6 (B=W, Re, Os) would be promising candidates as spintronic materials.

ChemInform Abstract: Nanostructured Mo‐Based Electrode Materials for Electrochemical Energy Storage

AbstractReview: 409 refs.

Growth of MoSi<sub>2</sub> by Molten Salt Technique Using Mo-Based Compounds

The hexagonal phase MoSi2 was grown using MoS2 powder as the source material by the molten salt method. The structural and morphological properties of the resultant silicides are characterized. As a comparison, the structural and compositional characterizations were made for the Mo-silicides grown on a Mo substrate and a Mo-Ti alloy compact. It was revealed that the homogeneity of the chemical composition of the silicides grown using MoS2 is improved, except for the existence of the Si source powders, though a small amount of S remained in the powder. The layered structure of MoS2 with a hexagonal crystalline structure would affect the growth morphology and enhance the formation of the hexagonal MoSi2. A simple growth procedure to fix the crystalline structure using an appropriate starting material under a preferable growth environment is proposed. [DOI: 10.1380/ejssnt.2012.55]

Structural stability and mutual transformations of molybdenum carbide, nitride and phosphide

The structural stability and transformations of Mo carbide, nitride and phosphide were investigated under various atmosphere conditions by X-ray diffraction (XRD). The results indicated that the order of structural stability of these Mo-based compounds was as follows: Mo 2N < Mo 2C < MoP. Both Mo 2C and Mo 2N can be transformed to MoP, whereas the reverse transformations did not occur. Noticeably, compared with those Mo sources containing oxygen, the use of Mo 2C/Mo 2N as Mo-source can produce finely dispersed MoP nanoparticles by the temperature-programmed reaction (TPR) method. The result was probably due to the fact that lower-levels H 2O generated during synthesis process can avoid strong hydrothermal sintering. The influence of formation energy had been considered and was found to relate to the structural stability and transformations of these Mo-based compounds.

Mo-Based Compounds for SiC-SiC Joints

New method to joint silicon carbide-based material was developed. It was based on mixture composed mainly by molybdenum silicides. This mixture was tested as brazing mixture with several types of silicon carbide-based material. Microstructural examination of the joint showed that brazing mixture reacted with substrate to form silicon carbide on the surface, while two different molybdenum silicides were identified inside the joint (MoSi{sub 2} and Mo{sub 4.8}Si{sub 3}C{sub 0.6}). Preliminary oxidation tests performed by means of TGA showed high oxidation resistance of this joint over 1000 C making it very promising for high temperature application like ceramic heat exchanger. (orig.)