Amorphous Molybdenum Research Articles

Propylene Metathesis over Molybdenum Silicate Microspheres with Dispersed Active Sites.

In this work, we demonstrate that amorphous and porous molybdenum silicate microspheres are highly active catalysts for heterogeneous propylene metathesis. Homogeneous molybdenum silicate microspheres and aluminum-doped molybdenum silicate microspheres were synthesized via a nonaqueous condensation of a hybrid molybdenum biphenyldicarboxylate-based precursor solution with (3-aminopropyl)triethoxysilane. The as-prepared hybrid metallosilicate products were calcined at 500 °C to obtain amorphous and porous molybdenum silicate and aluminum-doped molybdenum silicate microspheres with highly dispersed molybdate species inserted into the silicate matrix. These catalysts contain mainly highly dispersed MoOx species, which possess high catalytic activity in heterogeneous propylene metathesis to ethylene and butene. Compared to conventional silica-supported MoOx catalysts prepared via incipient wetness impregnation (MoIWI), the microspheres with low Mo content (1.5-3.6 wt %) exhibited nearly 2 orders of magnitude higher steady-state propylene metathesis rates at 200 °C, approaching site time yields of 0.11 s-1.

Hydrogen spillover in MoOxRh hierarchical nanosheets boosts alkaline HER catalytic activity

The low intrinsic catalytic performance and stability are the two dominant obstacles for alkaline hydrogen evolution reaction (HER) electrocatalyst. Herein, amorphous molybdenum oxide (MoOx) has been introduced to rhodium hierarchical nanosheets (MoOxRh-HNS) at atomic level efficiently reducing the barrier for hydrogen spillover from Rh to MoOx; thereby, MoOxRh-HNS exhibit superior HER activity with mass activity of 23.9 A mgnoble metal−1 at overpotential of 200 mV, which is boosted by 10.8- and 14.9-fold with relative to Rh-HNS and Pt/C in alkaline media, respectively. Meanwhile, Tafel slope is as low as 22.5 mV dec−1 for MoOxRh-HNS revealing a hydrogen-spillover-involving HER mechanism proved by the electrochemical test as well as operando electrochemical impedance spectroscopy. Density functional theory calculation indicates the adsorbed hydrogen on MoOxRh promotes the water dissociation and balances the hydrogen binding strength to a moderate level. Simultaneously, the hierarchical nanosheet structure contributes a robust HER stability at 500 mA cm−2. This work offers a new methodology to construct a high-performance alkaline HER electrocatalyst with industrial-level current density.

Amorphous molybdenum sulfide/polymeric carbon nitride composite with multiple interfaces for boosting photoelectrochemical water splitting performance

Rational interface design is an important way to improve the photoelectrochemical activity of catalysts in the sustainable energy and the environment. Herein, we simultaneously realize the construction of polymeric carbon nitride (PCN)-based homojunction (HPCN) and amorphous molybdenum sulfide (a-MoSx) /PCN heterojunction (a-MoSx/HPCN) in the composite catalyst system by one-pot calcination. The presence of a-MoSx was confirmed by X-ray photoelectron spectroscopy (XPS) and element mapping. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed the amorphous characteristics of molybdenum sulfide. The photoluminescence (PL) spectroscopy indicated that the photo-generated carrier recombination of composite is effectively inhibited after constructing the homojunction and loading molybdenum sulfide. The confined effect of the PCN matrix induced the formation of ultrafine size a-MoSx, and its co-catalyst function is prominent. The optimized composite catalyst showed obvious boosting activity in photoelectrochemical water slitting. Photocathode current of −161 μA cm−2 (0 V vs. RHE) and photoanode current of 10 μA cm−2 (1.23 V vs. RHE) are almost 12 and 5 times than single PCN homojunction under the irradiation of 25 W light-emitting diode light source. This boosted activity can be attributed to the multiple interface effect of the n-n junction of PCN/PCN and the p-n junction of a-MoSx/PCN as well as the co-catalysis of amorphous molybdenum sulfide. The synergies between various merits effectively improved the absorption of visible light, the separation of photogenerated electrons and holes, and the addition of more reactive sites. This paper provides a new idea for interface engineering of PCN-based composite photoelectrode.

Mott-Schottky heterojunctions of ultrafine Ni3Fe confined in amorphous molybdenum oxide for efficient and durable overall water electrolysis

NiFe alloys are highly active towards oxygen evolution reaction (OER), and it is important to further enhance their catalytic activity and working stability. Herein, a series of Mott-Schottky electrocatalysts of ultrafine Ni3Fe confined in amorphous molybdenum oxide (Ni3Fe/MoOx) are fabricated via hydrogenation treatment. The experimental results and computational calculations show that the amorphous MoOx improves the surface wettability, modulates the electronic structure, accelerates the charge transfer, favors the generation of NiOOH species, and facilitates the formation of the *OOH intermediates. The Ni3Fe/MoOx phase interfaces are engineered by controlling the MoOx content to optimize the catalytic activity, and the Ni3Fe/MoOx-0.5 catalyst with the optimized Mott-Schottky heterojunction exhibits the highest catalytic activity with an overpotential of 224 mV at 10 mA cm−2. Additionally, the amorphous MoOx layer greatly enhances the working stability of the Ni3Fe, showing almost no activity degradation in both two-electrode and three-electrode systems.

Ab Initio Molecular Dynamics Simulations of Amorphous Metal-Polysulfides as Cathode Materials for Lithium-Ion Batteries

Lithium-sulfur batteries offer a few advantages, including high energy density and low cost, compared to our current battery technology. However, various polysulfide intermediates formed during charging and discharging lead to the polysulfide shuttle effect, which hinders their commercial application. It has been shown that amorphous metal sulfides can reduce the formation of polysulfides, although a fundamental understanding is lacking. In this research, amorphous molybdenum and titanium sulfides with different sulfur concentrations were generated by the simulated melt-quench process using ab initio molecular dynamics simulations. Different numbers of lithium ions were added at the cathode/electrolyte interface to simulate the electrochemical conversion process in these materials. It is observed that the behavior of amorphous metal sulfides is different from that of sulfur, and the formation and diffusion of lithium polysulfides have been greatly reduced. Insights obtained from this work could help to develop new strategies to effectively inhibit the polysulfide shuttle effect.

Molybdenum oxysulfide MoOS2 as a hydrogen evolution catalyst: On the possible role of coordinated oxygen

Amorphous molybdenum oxysulfide MoOS2 obtained by acidification of aqueous MoO2S22− solutions was structurally characterized and tested as a catalyst for hydrogen evolution reaction (HER). Raman, XPS and XAS characterizations show that the solid contains oligomers based on the Mo(V) core Mo2O2S4 units, exposing dangling MoO bonds and connected via Mo-S-Mo bridges. MoOS2 demonstrated higher HER activity than reference catalysts including nanocrystalline MoS2, amorphous MoS3 and MoS2/MoO2 composite obtained by thermal decomposition of MoOS2. DFT calculations suggest that introduction of oxygen adjacent to the disulfide bridges in the molybdenum coordination does not change significantly the free energy of hydrogen desorption, as compared with fully sulfided MoxSy clusters. Higher HER catalytic activity of the MoOS2 material was explained by abundance of disulfide S22− moieties able to dissociate hydrogen and by fine dispersion of the material.

An Amorphous Molybdenum Polysulfide Cathode for Rechargeable Magnesium Batteries.

Rechargeable magnesium batteries (RMBs) attract research interest owing to the low cost and high reliability, but the design of cathode materials is the major difficulty of their development. The bivalent magnesium cation suffers from a strong interaction with the anion and is difficult to intercalate into traditional magnesium intercalation cathodes. Herein, an amorphous molybdenum polysulfide (a-MoSx ) is synthesized via a simple one-step solvothermal reaction and used as the cathode material for RMBs. The a-MoSx cathode provides a high capacity (185 mAh g-1 ) and a good rate performance (50 mAh g-1 at 1000 mA g-1 ), which are much superior compared with crystalline MoS2 and demonstrate the privilege of amorphous RMB cathodes. A mechanism study demonstrates both of molybdenum and sulfur undergo redox reactions and contribute to the capacity. Further optimizations indicate low-temperature synthesis would favor the magnesium storage performance of a-MoSx .

Mo2 S12 ]2- Complex as a Precursor for In-situ Generation of Molybdenum Sulfide Catalyst During the H2 Evolution.

For decades, the sulfido molybdenum complexes like [MoS4 ]2- , [Mo2 S12 ]2- , [Mo3 S13 ]2- have gained great attention because of their chemical versatility as well as their structural similarity to the edge-plan of the molybdenum disulfide (MoS2 ) which shows promising catalytic ability for the H2 generation. In this work, we report on the investigation of the dinuclear complex [Mo2 S12 ]2- in both organic and aqueous solution. We demonstrate that [Mo2 S12 ]2- is not intact during the H2 evolution catalysis when it is assayed as a homogeneous catalyst in an electrolyte solution (e. g. in DMF or water solvent) nor when it is immobilized on an electrode surface (e. g. mesoporous carbon black). It transforms into the polymeric amorphous molybdenum sulfide [MoS] which subsequently acts as an actual catalyst. We discuss on the possible [Mo2 S12 ]2- to [MoS] transformation mechanism by employing an arsenal of electrochemical analysis, spectroscopic analyses and microscopic analyses. Effects of the electrochemical operating conditions to the [Mo2 S12 ]2- to [MoS] transformation as well as to the chemical nature and the catalytic performance of the [MoS] product are also emphasized.

Field-induced reversible insulator-to-metal transition and the onset of ferroelectricity in molybdenum trioxide films

This paper presents the experimental evidence of reversible insulator–metal transition (IMT) in thin-film amorphous molybdenum trioxide (MoO3) induced by electric fields of just a few volts. The presence of oxygen vacancies in MoO3 is considered to play a significant role in the reported reversible IMT. The oxygen vacancies not only impact MoO3 stoichiometry but also the optical bandgap. The subthreshold slope for IMT in 10 nm-thick MoO3-based devices is 48.3 mV/decade, which represents a transition from an insulator to a metallic state, and the electric field threshold for such a transition was found to be equal to 0.034 V/Å. Following the IMT in MoO3, there are six orders of magnitude differences between the resistivity of the insulator state (27.5 M Ω at −9 V) and the metallic state (80 Ω between +5 and +9 V). In addition, we reported stabilization of a nanocrystalline hexagonal MoO3 (h-MoO3) phase in thicker MoO3 (150 nm-thick) in the presence of oxygen vacancies that behave as a wide bandgap (3.1 eV) ferroelectric semiconductor with a coercive field of about 50 kV/cm, a saturation polarization of about 30 μC/cm2, and a remanent polarization of about 10 μC/cm2. This ferroelectricity in nanocrystalline h-MoO3 (150 nm-thick) remains stable even after 8 months of storage of the sample in ambient conditions, with remanent polarization increasing up to 20 μC/cm2. These are unexpected results from MoO3.

Amorphous Molybdenum Disulfide Thin Film for Photocatalysis and Gas Sensing Applications

Amorphous Molybdenum Disulfide Thin Film for Photocatalysis and Gas Sensing Applications

Combining Highly Dispersed Amorphous MoS3 with Pt Nanodendrites as Robust Electrocatalysts for Hydrogen Evolution Reaction.

Surface modification of electrocatalysts to obtain new or improved electrocatalytic performance is currently the main strategy for designing advanced nanocatalysts. In this work, highly dispersed amorphous molybdenum trisulfide-anchored Platinum nanodendrites (denoted as Pt-a-MoS3 NDs) are developed as efficient hydrogen evolution electrocatalysts. The formation mechanism of spontaneous in situ polymerization MoS4 2- into a-MoS3 on Pt surface is discussed in detail. It is verified that the highly dispersed a-MoS3 enhances the electrocatalytic activity of Pt catalysts under both acidic and alkaline conditions. The potentials at the current density of 10mA cm-2 (η10 ) in 0.5msulfuric acid (H2 SO4 ) and 1mpotassium hydroxide (KOH) electrolyte are -11.5 and -16.3mV, respectively, which is significantly lower than that of commercial Pt/C (-20.2mV and -30.7mV). This study demonstrates that such high activity benefits from the interface between highly dispersed a-MoS3 and Pt sites, which act as the preferred adsorption sites for the efficient conversion of hydrion (H+ ) to hydrogen (H2 ). Additionally, the anchoring of highly dispersed clusters to Pt substrate greatly enhances the corresponding electrocatalytic stability.

Boosting highly active defect MoV sites for amorphous molybdenum sulfide from catalyst-substrate effect toward efficient hydrogen evolution

Simultaneously achieving abundant active sites and high intrinsic activity is a major challenge to developing highly efficient MoSx-based materials for hydrogen evolution reaction (HER) since there might be a contradictory relationship between loading amounts and intrinsic activity of amorphous molybdenum sulfide (MoSx). Herein, we have for the first time demonstrated a strategy for the electrochemical oxidation of carbon cloth (CC) followed by mild acidification employing (NH4)2MoS4 as the precursor to boost highly active defect MoV sites (DMSs) for MoSx toward efficient HER. In our strategy, a high potential at the anode results in a high concentration of O-containing groups on the surface of CC, which favors loading [Mo3S13]2− clusters or MoSx on CC (MoSx/CC) for achieving abundant DMSs. Impressively, such oxidation does not disrupt the sp2 hybridized carbon of the substrate. Hence, the excellent conductivity of CC guarantees high intrinsic activity of the as-synthesized MoSx/CC samples. On the other hand, excessively high potential stimulates excessive growth and accumulation of [Mo3S13]2− clusters for further increasing loading amounts of MoSx. Nevertheless, such loading is at the expense of DMSs, indicating a great decrease in the quantity of active sites. Furthermore, excessive oxidation leads to unsatisfactory conductivity of CC, owing to breaking the sp2-conjugated system of CC. At the same time, the theoretical prediction and experimental investigation reveal that the intrinsic activity of DMSs will significantly decrease at excessively high potential. By regulating electrochemical potential to 1.8 V, a balance between the loading amounts of MoSx and intrinsic activity of MoSx/CC is achieved to boost highly active DMSs for efficient HER. Consequently, the optimal MoSx/CC exhibit the most excellent HER activity among all counterparts, accompanied by reliable stability. This work presents a new strategy for realizing abundant active sites with high intrinsic activity for MoSx-based materials toward efficient HER from the catalyst-substrate effect.

Covalency triggers high catalytic activity of amorphous molybdenum oxides for oxidative desulfurization

Covalency triggers high catalytic activity of amorphous molybdenum oxides for oxidative desulfurization

All-fiber device for single-photon detection

Fiber components form the standard not only in modern telecommunication but also for future quantum information technology. For high-performance single-photon detection, superconducting nanowire single-photon detectors (SPDs) are typically fabricated on a silicon chip and fiber-coupled for easy handling and usage. The fiber-to-chip interface hinders the SPD from being an all-fiber device for full utilization of its excellent performance. Here, we report a scheme of SPD that is directly fabricated on the fiber tip. A bury-and-planar fabrication technique is developed to improve the roughness of the substrate for all-fiber detectors’ performance for single-photon detection with amorphous molybdenum silicide (MoSi) nanowires. The low material selectivity and universal planar process enable fabrication and packaging on a large scale. Such a detector responds to a broad wavelength range from 405 nm to 1550 nm at a dark count rate of 100 cps. The relaxation time of the response pulse is ~ 15 ns, which is comparable to that of on-chip SPDs. Therefore, this device is free from fiber-to-chip coupling and easy packaging for all-fiber quantum information systems.

MnWO4 nanorods embedded into amorphous MoS microsheets in 2D/1D MoS /MnWO4 S–scheme heterojunction for visible-light photocatalytic water oxidation

• A novel MoS x /MnWO 4 hybrid was constructed for boosting photocatalytic O 2 evolution. • A charge accumulation interlayer between MoS x and MnWO 4 accelerates charge transportation. • The S–scheme heterostructure promotes efficiencies of sunlight utilization and charge separation. In this work, a novel amorphous molybdenum disulfide/manganese tungstate (MoS x /MnWO 4 ) hybrid in S–scheme heterojunction was designed and synthesized for photocatalytic water splitting to oxygen generation under visible light. In the hybrids, a strong chemical interlayer between amorphous MoS x microsheets and MnWO 4 nanorods was created by partial substitution of Mo for W, which increases charge transportation efficiency by reducing charge transfer barrier, verified by the computational density functional theory (DFT) and photoelectrochemical tests. A 0.5 wt% MoS x /MnWO 4 system (2D/1D) displayed a remarkable enhancement in photocatalytic activity of O 2 evolution, up to 267.8 μmol g –1 under visible light illumination (> 420 nm). The formed S–scheme heterojunction structure efficiently promotes the utilization of solar light and separation efficiency of photo-generated charge carriers, leading to the improvement of photocatalytic water oxidation performance.

Guaiacol to Aromatics: Efficient Transformation over In Situ-Generated Molybdenum and Tungsten Oxides

The development of catalysts for the hydrodeoxygenation of bio-based feedstocks is an important step towards the production of fuels and chemicals from biomass. This paper describes in situ-generated bulk molybdenum and tungsten oxides in the hydrodeoxygenation of the lignin-derived compound guaiacol. The catalysts obtained were studied using powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high-resolution transition electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, and Raman spectroscopy. The use of metal carbonyls as precursors was shown to promote the formation of amorphous molybdenum oxide and crystalline tungsten phosphide under hydrodeoxygenation conditions. The catalysts’ activity was investigated under various reaction conditions (temperature, H2 pressure, solvent). MoOx was more active in the partial and full hydrodeoxygenation of guaiacol at temperatures of 200–380 °C (5 MPa H2, 6 h). However, cyclohexane, which is an undesirable product, was formed in significant amounts using MoOx (5 MPa H2, 6 h), while WOx was more selective to aromatics. When using dodecane as a solvent (380 °C, 5 MPa H2, 6 h), the benzene-toluene-xylenes fraction was obtained with a 96% yield over the WOx catalyst.

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Fabrication of inverse opal molybdenum sulfide and its use as a catalyst for H2 evolution.

Amorphous molybdenum sulfide (MoSx) and crystalline molybdenum disulfide (MoS2) are attractive noble-metal-free electrocatalysts for the H2 evolution reaction from water. Their actual activities depend on the quantity of active sites which are exposed to the electrolyte, which in turn, is influenced by their specific electrochemical surface area. Herein we report on the fabrication of regular inverse opal MoSx and MoS2 films by employing polystyrene nanoparticles with diameters in the range of 30-90 nm as hard templates. The use of these catalysts for the H2 evolution reaction in an acidic electrolyte solution is also presented. Impacts of the regular porous structure, the film thickness as well as the chemical nature of the catalyst (MoS2versus MoSx) are discussed. It shows a catalytically-effective-thickness of ca. 300 nm where the electrolyte can fully penetrate the catalyst macropores, thus all the catalytic active sites can be exposed to the electrolyte to achieve the maximal catalytic operation.

Composite structure of a-MoSX@Ni9S8/NF nanoflower rods for efficient HER under a thermal field

The a-MoSX@Ni9S8/NF was prepared by NF sulfidation followed by electrochemical deposition. With the assistance of a thermal field, the interfacial charge transfer efficiency can be increased, resulting in an improvement of HER performance of the a-MoSX@Ni9S8/NF.

Electrodepositing amorphous molybdenum oxides for aqueous NH4+ storage.

The limited choice of anode materials always challenges the development of high performance aqueous ammonium-ion batteries (AAIBs). Herein, we fabricate amorphous molybdenum oxide (MoOx) materials and study the NH4+ storage performances. The results indicate that the optimized electrode exhibits high gravimetric/areal capacities of 175 mA h g-1/1.30 mA h cm-2, outperforming state-of-the-art anode materials for AAIBs. Our findings indicate that the valence state of Mo and the Mo-O-H content in MoOx synergistically control the NH4+ storage performances, offering new understanding for rational design of MoOx materials for energy storage applications.