Showing papers on "Molybdenum published in 2021"

Single atom Ru doping 2H-MoS2 as highly efficient hydrogen evolution reaction electrocatalyst in a wide pH range

Abstract: It’s a huge challenge to develop a hydrogen evolution reaction (HER) catalyst with a wide pH range. Herein, single atom ruthenium-doped molybdenum disulfide with high 2H phase content (Ru@2H-MoS2) catalysts are synthesized by a two-step hydrothermal method, they are named as Ru0.05@2H-MoS2, Ru0.10@2H-MoS2 and Ru0.12@2H-MoS2 based on the amount of Ru (mmol) added to 2H-MoS2. The results indicate that single atom Ru substituting for molybdenum atom leads to more active centers and some vacancies, thus improving the property of 2H-MoS2 for HER. Among them, Ru0.10@2H-MoS2 shows the lowest HER overpotentials of 137, 51 and 168 mV at 10 mA cm-2 in 1.0 M PBS, 1.0 M KOH and 0.5 M H2SO4, respectively. The density functional theory (DFT) further indicates that the Ru@2H-MoS2 has a lower Gibbs free energy of H-adsorption (ΔGH*). Therefore, this study offers an effective way to reasonably design and synthesize efficient MoS2-based catalysts in a wide pH range.

From NiMoO4 to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy.

Abstract: Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. The development and evaluation of effective catalysts are here imperative; however, due to the inclusion of elements with different redox properties and reactivity, these materials undergo dynamical changes and phase transformations during the reaction conditions. NiMoO4 is currently investigated among other metal oxides as a promising noble metal free catalyst for the oxygen evolution reaction. Here we show that at applied bias, NiMoO4·H2O transforms into γ-NiOOH. Time resolved operando Raman spectroscopy is utilized to follow the potential dependent phase transformation and is collaborated with elemental analysis of the electrolyte, confirming that molybdenum leaches out from the as-synthesized NiMoO4·H2O. Molybdenum leaching increases the surface coverage of exposed nickel sites, and this in combination with the formation of γ-NiOOH enlarges the amount of active sites of the catalyst, leading to high current densities. Additionally, we discovered different NiMoO4 nanostructures, nanoflowers, and nanorods, for which the relative ratio can be influenced by the heating ramp during the synthesis. With selective molybdenum etching we were able to assign the varying X-ray diffraction (XRD) pattern as well as Raman vibrations unambiguously to the two nanostructures, which were revealed to exhibit different stabilities in alkaline media by time-resolved in situ and operando Raman spectroscopy. We advocate that a similar approach can beneficially be applied to many other catalysts, unveiling their structural integrity, characterize the dynamic surface reformulation, and resolve any ambiguities in interpretations of the active catalyst phase.

Molybdenum nitride@porous carbon, derived from phosphomolybdic acid loaded metal-azolate framework-6: A highly effective catalyst for oxidative desulfurization

Abstract: A new composite of the metal-azolate framework (MAF-6) and phosphomolybdic acid (PMA), named PMA(x)@MAF-6, was prepared. Well-dispersed (size: ∼4 nm) Mo2N, on porous carbon, was prepared without external nitrogen sources firstly via carbonization of the PMA(x)@MAF-6. The prepared catalyst was applied in oxidative desulfurization (ODS) with H2O2 as a greenoxidant and showed remarkable performance. A selected catalyst Mo2N@C-3 had the highest turnover frequency (30 h-1, around 2–3 times of the best catalyst so far) with the lowest activation energy (27.8 kJ·mol−1) in ODS (with H2O2) among any molybdenum-based catalysts. The ODS over Mo2N@C was explained by a non-radical mechanism via active Mo-peroxo species. Moreover, the high activity of the catalyst is partly because of the contribution of nitrogen in Mo2N. The studied catalyst could be easily recycled by solvent washing. Therefore, Mo2N, especially supported on carbon (like Mo2N@C), could be firstly suggested as a promising catalyst in ODS.

Highly active and stable nickel–molybdenum nitride (Ni2Mo3N) electrocatalyst for hydrogen evolution

Abstract: This paper reports a highly active and stable nonprecious metal electrocatalyst based on bimetallic nanoscale nickel molybdenum nitride developed for the hydrogen evolution reaction (HER). A composite of 7 nm Ni2Mo3N nanoparticles grown on nickel foam (Ni2Mo3N/NF) was prepared through a simple and economical synthetic method involving one-step annealing of Ni foam, MoCl5, and urea without a Ni precursor. The Ni2Mo3N/NF exhibits high activity with low overpotential (η10 of 21.3 mV and η100 of 123.8 mV) and excellent stability for the HER, achieving one of the best performances among state-of-the-art transition metal nitride based catalysts in alkaline media. Supporting density functional theory (DFT) calculations indicate that N sites in Ni2Mo3N with a N–Mo coordination number of four have a hydrogen adsorption energy close to that of Pt and hence may be responsible for the enhanced HER performance.

Selective Oxidation of Anilines to Azobenzenes and Azoxybenzenes by a Molecular Mo Oxide Catalyst.

Abstract: Aromatic azo compounds, which play an important role in pharmaceutical and industrial applications, still face great challenges in synthesis. Herein, we report a molybdenum oxide compound, [N(C4 H9 )4 ]2 [Mo6 O19 ] (1), catalyzed selective oxidation of anilines with hydrogen peroxide as green oxidant. The oxidation of anilines can be realized in a fully selectively fashion to afford various symmetric/asymmetric azobenzene and azoxybenzene compounds, respectively, by changing additive and solvent, avoiding the use of stoichiometric metal oxidants. Preliminary mechanistic investigations suggest the intermediacy of highly active reactive and elusive Mo imido complexes.

Hybrid heterojunction of molybdenum disulfide/single cobalt atoms anchored nitrogen, sulfur-doped carbon nanotube /cobalt disulfide with multiple active sites for highly efficient hydrogen evolution

Abstract: A multicomponent 3D monolithic electrode was designed that consists of Co single atoms anchored/nitrogen, sulfur co-doped carbon nanotubes (CoSAs-NS-CNTs) on carbon cloth (CC) with ultra-thin MoS2 nanosheets externally decorated and ultra-small CoS2 nanodots internally confined (MoS2/CoSAs-NS-CNTs@CoS2/CC) to form a hybrid heterojunction electrode with double interfaces as vectorial electron transport pathways for promoting electrocatalytic performance. The integration of well-distributed MoS2 nanosheets and CoS2 nanodots linked with Co single atoms anchored/N, S co-doped CNTs endows abundant multiple active sites, outstanding conductivity, and the downshifted d-band center with a thermodynamically favorable hydrogen adsorption free energy (ΔGH*) for effectively catalyzing hydrogen evolution reaction (HER). As a result, the optimal MoS2/CoSAs-NS-CNTs@CoS2/CC electrode exhibits outstanding HER performance with overpotentials of 72 and 56 mV at 10 mA cm−2 and small Tafel slopes of 59.4 and 43.2 mV dec-1 in acidic and alkaline solutions, respectively, outperforming most of the previously reported molybdenum/cobalt sulfide-based electrocatalysts. Moreover, the electrode also displays good durability and stability reflected from the small decrease on activity after 5000 CV cycles and nearly no decay in current density after electrolysis for 20 h.

Direct methane activation by atomically thin platinum nanolayers on two-dimensional metal carbides

Abstract: Efficient and direct conversion of methane to value-added products has been a long-term challenge in shale gas applications. Here, we show that atomically thin nanolayers of Pt with a single or double atomic layer thickness, supported on a two-dimensional molybdenum titanium carbide (MXene), catalyse non-oxidative coupling of methane to ethane/ethylene (C2). Kinetic and theoretical studies, combined with in-situ spectroscopic and microscopic characterizations, demonstrate that Pt nanolayers anchored at the hexagonal close-packed sites of the MXene support can activate the first C–H bond of methane to form methyl radicals that favour desorption over further dehydrogenation and thus suppress coke deposition. At 750 °C and 7% methane conversion, the catalyst runs for 72 hours of continuous operation without deactivation and exhibits >98% selectivity towards C2 products, with a turnover frequency of 0.2–0.6 s−1. Our findings provide insights into the design of highly active and stable catalysts for methane activation and create a platform for developing atomically thin supported metal catalysts. The challenge in non-oxidative coupling of methane lies in the activation of the first C–H bond while avoiding further dehydrogenations, which lead to the formation of coke. Here, atomically thin platinum nanolayers on two-dimensional molybdenum titanium carbides are reported as a superior catalyst for this reaction owing to reduced coke formation.

The age of bioinspired molybdenum-involved nanozymes: Synthesis, catalytic mechanisms, and biomedical applications

Abstract: Molybdenum (Mo), as a nontoxic and low‐cost transition metal, has been employed for synthesis of various Mo‐based nanomaterials with unique structures and physicochemical features to achieve various properties. Especially, bioinspired Mo‐based nanomaterials show great potential for the construction of novel nanozyme catalysts due to their variable oxidation states. Overcoming drawbacks of natural enzymes, bioinspired Mo‐based nanozymes not only provide effective catalytic sites or multivalent elements to mimic natural enzymes, but also present multiple functions for interfacing with various biomicroenvironments. Construction of vast Mo‐based nanozymes has attracted enormous interest in biomedicine. Exogenous/endogenous stimuli enable the user to tailor the catalytic activities of Mo‐based nanozymes. Additionally, tunable physicochemical properties also have a significant influence on their enzyme‐like activity. In this review, we comprehensively summarize typical synthesis strategies, catalytic mechanism, and types of enzyme‐like activity of the bioinspired Mo‐based nanozymes. We mainly highlight desired merits of bioinspired Mo‐based nanozymes related to tunable enzyme‐like activity, stability, and multifunctionality through regulating their physicochemical properties. Furthermore, we intend to discuss their biomedical applications in biosensing and detection, oncotherapy, and combating bacteria. Finally, current challenges and future perspectives of the Mo‐based nanozymes are also proposed.

Nitrogen-Doped Metallic MoS2 Derived from a Metal-Organic Framework for Aqueous Rechargeable Zinc-Ion Batteries.

Abstract: Molybdenum disulfide (MoS2) has been extensively studied as a potential storage material for batteries. However, the electrochemical performance of MoS2 is far from ideal, and it exhibits severe activity fading resulting from its low electronic conductivity. The present work synthesizes nitrogen (N)-doped 1T MoS2 nanoflowers made of ultrathin nanosheets via the one-step hydrothermal sulfurization of a molybdenum-based metal-organic framework precursor. The resulting metallic phase shows improved conductivity and hydrophilicity, and characterization demonstrates that N doping effectively expands the interlayer spacing and increases the concentration of sulfur vacancies serving as defects. This material demonstrates high rate performance and good cycling stability when used as the cathode in an aqueous rechargeable zinc-ion battery (ARZIB). Its performance is superior to those of pure 1T MoS2 and 2H MoS2 synthesized with MoO3 as the molybdenum source. Ex situ X-ray photoelectron spectroscopy and X-ray diffraction analyses are performed to explore the reaction mechanism during charging and discharging of the N-doped 1T MoS2. A three-cell series ARZIB system containing this material is used to power five light-emitting diodes to confirm the possible practical applications of this technology.

Ammonium Intercalation Induced Expanded 1T-Rich Molybdenum Diselenides for Improved Lithium Ion Storage.

Abstract: Transition metal dichalcogenides (TMDs), particularly molybdenum diselenides (MoSe2), have the merits of their unique two-dimensional (2D) layered structures, large interlayer spacing (∼0.64 nm), good electrical conductivities, and high theoretical capacities when applied in lithium-ion batteries (LIBs) as anode materials. However, MoSe2 remains suffering from inferior stability as well as unsatisfactory rate capability because of the unavoidable volume expansion and sluggish charge transport during lithiation-delithiation cycles. Herein, we develop a simultaneous reduction-intercalation strategy to synthesize expanded MoSe2 (e-MoSe2) with an interlayer spacing of 0.98 nm and a rich 1T phase (53.7%) by rationally selecting the safe precursors of ethylenediamine (NH2C2H4NH2), selenium dioxide (SeO2), and sodium molybdate (Na2MoO4). It is noteworthy that NH2C2H4NH2 can effectively reduce SeO2 and MoO42- forming MoSe2 nanosheets; in the meantime, the generated ammonium (NH4+) efficiently intercalates between MoSe2 layers, leading to charge transfer, thus stabilizing 1T phases. The obtained e-MoSe2 exhibits high capacities of 778.99 and 611.40 mAh g-1 at 0.2 and 1 C, respectively, together with excellent cycling stability (retaining >90% initial capacity at 0.2 C over 100 charge-discharge cycles). It is believed that the material design strategy proposed in this paper provides a favorable reference for the synthesis of other transition metal selenides with improved electrochemical performance for battery applications.

Investigation of microstructure, mechanical and machinability properties of Mo-added steel produced by powder metallurgy method

Abstract: This study presents the impact of molybdenum (Mo) inclusion on microstructure, mechanical, and machinability behavior of steels manufactured with powder metallurgy (PM) approach. PM steel samples with different molybdenum ratios were pressed at 750-MPa pressing pressure and sintered in the atmosphere-controlled tube furnace at 1400°C for 1 h. While particle size and distribution of phases of PM steels with different molybdenum ratios were determined by optical microscope, mechanical properties were determined by applying tensile test. The results were observed that 3% Mo weight-added steel displayed the maximum yield and tensile strength. In addition, the machinability properties of 3% Mo-added steel, which has the highest yield and tensile strength, were investigated. In this work, we researched the thrust force and surface roughness as machinability output, and drilling parameters on the output were determined by utilizing analysis of variance. Finally, SEM images were taken from the inner surfaces of the machined holes and the fractured surfaces from tensile test, and information about the machinability of this alloy produced with PM was presented. As a result, an increase in yield and tensile strength and a reduction in strain were identified with the increase in Mo content. Moreover, the coated cutting tools are better on the machining output than the uncoated cutting tools in terms of green environment. The most important factors on the Fz and Ra are the coating condition and the feed rate with 56.53% and 43.62% PCR, respectively.

Recent Advances in Molybdenum-Based Materials for Lithium-Sulfur Batteries

Abstract: Lithium-sulfur (Li-S) batteries as power supply systems possessing a theoretical energy density of as high as 2600 Wh kg−1 are considered promising alternatives toward the currently used lithium-ion batteries (LIBs). However, the insulation characteristic and huge volume change of sulfur, the generation of dissolvable lithium polysulfides (LiPSs) during charge/discharge, and the uncontrollable dendrite formation of Li metal anodes render Li-S batteries serious cycling issues with rapid capacity decay. To address these challenges, extensive efforts are devoted to designing cathode/anode hosts and/or modifying separators by incorporating functional materials with the features of improved conductivity, lithiophilic, physical/chemical capture ability toward LiPSs, and/or efficient catalytic conversion of LiPSs. Among all candidates, molybdenum-based (Mo-based) materials are highly preferred for their tunable crystal structure, adjustable composition, variable valence of Mo centers, and strong interactions with soluble LiPSs. Herein, the latest advances in design and application of Mo-based materials for Li-S batteries are comprehensively reviewed, covering molybdenum oxides, molybdenum dichalcogenides, molybdenum nitrides, molybdenum carbides, molybdenum phosphides, and molybdenum metal. In the end, the existing challenges in this research field are elaborately discussed.

Molybdenum impregnated g-C 3 N 4 nanotubes as potentially active photocatalyst for renewable energy applications.

Abstract: Molybdenum (Mo) impregnated g-C3N4 (Mo-CN) nanotubes are fabricated via a thermal/hydrothermal process to augment photoelectrochemical properties during solar-driven water-splitting (SDWS) reactions. Graphitic-C3N4 is an attractive material for photocatalysis because of its suitable band energy, high thermal and chemical stability. The FE-SEM and HR-TEM comprehend the nanotube-like morphology of Mo-CN. The spectroscopic characterization revealed bandgap energy of 2.63 eV with high visible-light activity. The x-ray diffraction of pristine g-C3N4 and Mo-CN nanotubes discloses the formation of triazine-based nanocrystalline g-C3N4, which remains stable during hydrothermal impregnation of Mo. Furthermore, Mo-CN nanotubes possess high sp2-hybridized nitrogen content, and metallic/oxidized Mo nanoparticles (in a ratio of 1:2) are impregnated into g-C3N4. The XPS analysis confirms C, N, and Mo for known atomic and oxidation states in Mo-CN. Furthermore, high photocurrent efficiency (~ 5.5 mA/cm2) is observed from 5%-Mo-CN nanotubes. That displays efficient SDWS by 5%-Mo-CN nanotubes than other counterparts. Impedance spectroscopy illustrated the lowest charge transfer resistance (Rct) of 5%-Mo-CN nanotubes, which further confirms the fast electron transfer kinetics and efficient charge separation resulting in high photocurrent generation. Hence, 5%Mo-CN composite nanotubes can serve as a potential photocatalytic material for viable solar-driven water splitting.

Synthesis and Characterization of a Well-Defined Carbon Nanohorn-Supported Molybdenum Dioxo Catalyst by SMART-EM Imaging. Surface Structure at the Atomic Level

Abstract: The molybdenum dioxo catalyst CNH/MoO2 is prepared via direct grafting of (dme)MoO2Cl2 (dme = 1,2-dimethoxyethane) onto the graphitic surfaces of carbon nanohorn (CNH) substrates. The structure of ...

Catalytic Reduction of N2 to Borylamine at a Molybdenum Complex

Abstract: Catalytic formation of borylamines from atmospheric N2 is achieved for the first time using a series of homogenous (triphosphine)Mo complexes. Stepwise functionalization of the (triphosphine)Mo-nitrido complex with chloroborane led to the synthesis of the imido complex. Electrochemical characterization of the (PPP)Mo-nitrido and (PPP)Mo-borylimido complexes showed that the latter is much more easily reduced.