Porous Molybdenum Carbide Nanostructures Synthesized on Carbon Cloth by CVD for Efficient Hydrogen Production

Molybdenum carbide (Mo₂C) is a promising noble-metal-free electrocatalyst for the hydrogen evolution reaction (HER), due to its structural and electronic merits, such as high conductivity, metallic band states and wide pH applicability. Here, a simple CVD process was developed for synthesis of a Mo₂C on carbon cloth (Mo₂C@CC) electrode with carbon cloth as carbon source and MoO₃ as the Mo precursor. XRD, Raman, XPS and SEM results of Mo₂C@CC with different amounts of MoO₃ and growth temperatures suggested a two-step synthetic mechanism, and porous Mo₂C nanostructures were obtained on carbon cloth with 50 mg MoO₃ at 850 °C (Mo₂C-850(50)). With the merits of unique porous nanostructures, a low overpotential of 72 mV at current density of 10 mA cm⁻² and a small Tafel slope of 52.8 mV dec⁻¹ was achieved for Mo₂C-850(50) in 1.0 m KOH. The dual role of carbon cloth as electrode and carbon source resulted into intimate adhesion of Mo₂C on carbon cloth, offering fast electron transfer at the interface. Cyclic voltammetry measurements for 5000 cycles revealed that Mo₂C@CC had excellent electrochemical stability. This work provides a novel strategy for synthesizing Mo₂C and other efficient carbide electrocatalysts for HER and other applications, such as supercapacitors and lithium-ion batteries.     

Synthesis of Ultrathin and Grid-Structural Carbon Nanosheets Coupled with Mo2C for Electrocatalytic Hydrogen Production

Molybdenum carbide possessing a Pt-like d-band electronic structure is considered as one of potential candidates of electrocatalysts and it shows intrinsic catalytic property. However, a high carbonizing temperature easily leads to the coalescence of nanoparticles (NPs). Here, we propose a simple sol-gel route to achieve high dispersity of carbide NPs by designing a Mo-involved xerogel. The results show that molybdenum carbide NPs are dispersed and anchored on the nitrogen-doped carbon nanosheets (Mo₂C@NC). Ultrathin carbon layers resemble graphene and the network structures act as a support of carbide NPs, which can hinder NPs’ coalescence effectively. Nanpoparticles cross-coupled on network-structure nanosheets display the grid shapes. Electrochemical studies indicate that Mo₂C@NC material exhibits outstanding hydrogen evolution performance in alkaline electrolyte.     

A facile preparation of nanocrystalline Mo2C from graphite or carbon nanotubes

Nanocrystalline Mo₂C powders were successfully synthesized at 500 °C by reacting molybdenum chloride (MoCl₅) with C (graphite or carbon nanotube) in metallic sodium medium. X-ray powder diffractometer (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscope (XPS) and surface area analyzer (BET method) were used to characterize the samples. Experiments reveal that the carbon source used for the carbide synthesis has a great effect on the particle size and the surface area of the samples. When micro-sized graphite was used as C source the obtained nanocrystalline Mo₂C powder consists of particles of 30∼100 nm, with a surface area of 2.311 m²/g. When carbon nanotubes were used as C source, the as-synthesized Mo₂C sample is composed of particles of 20∼50 nm, with a surface area of 23.458 m²/g, which is an order of magnitude larger than that of the carbide prepared from the graphite.     

Bottom-up Design of Bimetallic Cobalt–Molybdenum Carbides/Oxides for Overall Water Splitting

Earth-abundant transition-metal-based catalysts for electrochemical water splitting are critical for sustainable energy schemes. In this work, we use a rational design method for the synthesis of ultrasmall and highly dispersed bimetallic CoMo carbide/oxide particles deposited on graphene oxide. Thermal conversion of the molecular precursors [H₃PMo₁₂O₄₀], Co(OAc)₂ ⋅ 4 H₂O and melamine in the presence of graphene oxide gives the mixed carbide/oxide (Co₆Mo₆C₂/Co₂Mo₃O₈) nanoparticle composite deposited on highly dispersed, N,P-doped carbon. The resulting composite shows outstanding electrocatalytic water-splitting activity for both the oxygen evolution and hydrogen evolution reaction, and superior performance to reference samples including commercial 20 % Pt/C & IrO₂. Electrochemical and other materials analyses indicate that Co₆Mo₆C₂ is the main active phase in the composite, and the N,P-doping of the carbon matrix increases the catalytic activity. The facile design could in principle be extended to multiple bimetallic catalyst classes by tuning of the molecular metal oxide precursor.     

Engineering Non‐sintered,Metal‐Terminated Tungsten Carbide Nanoparticles for Catalysis

Transition‐metal carbides (TMCs) exhibit catalytic activities similar to platinum group metals (PGMs), yet TMCs are orders of magnitude more abundant and less expensive. However, current TMC synthesis methods lead to sintering, support degradation, and surface impurity deposition, ultimately precluding their wide‐scale use as catalysts. A method is presented for the production of metal‐terminated TMC nanoparticles in the 1–4 nm range with tunable size, composition, and crystal phase. Carbon‐supported tungsten carbide (WC) and molybdenum tungsten carbide (Mo_xW_1−xC) nanoparticles are highly active and stable electrocatalysts. Specifically, activities and capacitances about 100‐fold higher than commercial WC and within an order of magnitude of platinum‐based catalysts are achieved for the hydrogen evolution and methanol electrooxidation reactions. This method opens an attractive avenue to replace PGMs in high energy density applications such as fuel cells and electrolyzers.     

Engineering Non‐sintered,Metal‐Terminated Tungsten Carbide Nanoparticles for Catalysis

Transition‐metal carbides (TMCs) exhibit catalytic activities similar to platinum group metals (PGMs), yet TMCs are orders of magnitude more abundant and less expensive. However, current TMC synthesis methods lead to sintering, support degradation, and surface impurity deposition, ultimately precluding their wide‐scale use as catalysts. A method is presented for the production of metal‐terminated TMC nanoparticles in the 1–4 nm range with tunable size, composition, and crystal phase. Carbon‐supported tungsten carbide (WC) and molybdenum tungsten carbide (Mo_xW_1?xC) nanoparticles are highly active and stable electrocatalysts. Specifically, activities and capacitances about 100‐fold higher than commercial WC and within an order of magnitude of platinum‐based catalysts are achieved for the hydrogen evolution and methanol electrooxidation reactions. This method opens an attractive avenue to replace PGMs in high energy density applications such as fuel cells and electrolyzers.     

Mo2C/Reduced Graphene Oxide Composites with Enhanced Electrocatalytic Activity and Biocompatibility for Microbial Fuel Cells

A simple, cost-effective strategy was developed to effectively improve the electron transfer efficiency as well as the power output of microbial fuel cells (MFCs) by decorating the commercial carbon paper (CP) anode with an advanced Mo₂C/reduced graphene oxide (Mo₂C/RGO) composite. Benefiting from the synergistic effects of the superior electrocatalytic activity of Mo₂C, the high surface area, and prominent conductivity of RGO, the MFC equipped with this Mo₂C/RGO composite yielded a remarkable output power density of 1747±37.6 mW m⁻², which was considerably higher than that of CP-MFC (926.8±6.3 mW m⁻²). Importantly, the composite also facilitated the formation of 3D hybrid biofilm and could effectively improve the bacteria–electrode interaction. These features resulted in an enhanced coulombic efficiency up 13.2 %, nearly one order of magnitude higher than that of the CP (1.2 %).     

Ultrafine Molybdenum Carbide Nanoparticles Composited with Carbon as a Highly Active Hydrogen‐Evolution Electrocatalyst

The replacement of platinum with non‐precious‐metal electrocatalysts with high efficiency and superior stability for the hydrogen‐evolution reaction (HER) remains a great challenge. Herein, we report the one‐step synthesis of uniform, ultrafine molybdenum carbide (Mo₂C) nanoparticles (NPs) within a carbon matrix from inexpensive starting materials (dicyanamide and ammonium molybdate). The optimized catalyst consisting of Mo₂C NPs with sizes lower than 3 nm encapsulated by ultrathin graphene shells (ca. 1–3 layers) showed superior HER activity in acidic media, with a very low onset potential of ?6 mV, a small Tafel slope of 41 mV dec^?1, and a large exchange current density of 0.179 mA cm^?2, as well as good stability during operation for 12 h. These excellent properties are similar to those of state‐of‐the‐art 20 % Pt/C and make the catalyst one of the most active acid‐stable electrocatalysts ever reported for HER.     

Studies on molybdenum carbide supported HZSM-5 (Si/Al = 23, 30, 50 and 80) catalysts for aromatization of methane

Here in, for the first time we are reporting molybdenum carbide reduction into metallic molybdenum during methane aromatization on HZSM-5 (Si/Al ratio = 23, 30, 50 and 80) at methane space velocity of 1800 mL.g_cat⁻.h⁻. Benzene yield was influenced by the surface metallic molybdenum through the non-aromatic carbon deposits formation via linear hydrocarbons degradation on HZSM-5 with fewer acidity (Si/Al ratio = 30, 50 and 80). Our XPS analysis results demonstrated improved surface metallic molybdenum in spent Mo₂C/HZSM-5 = 80 (0.71 atom. %) and 50 (0.54 atom. %) samples over Mo₂C/HZSM-5 = 30 (0.33 atom. %) and 23 (0.20 atom. %) samples. Furthermore, HR-TEM and FFT analysis images clearly established fine distribution of distorted spherical shaped Mo₂C particles with 6–14 nm size in spent Mo₂C/HZSM-5 = 23. On the other hand, Mo₂C particle size was increased upto 22 nm in Mo₂C/HZSM-5 = 80. The ease reduction of Mo₂C into metallic molybdenum and aggregation of Mo₂C particles in spent higher Si to Al ratio (50 and 80) samples was associated with weak interactions between Mo₂C and the HZSM-5 with fewer acidity. At 700 °C, the order of benzene yield as follows: Mo₂C/HZSM-5 = 80 (2.2%) < Mo₂C/HZSM-5 = 50 (3.25%) < Mo₂C/HZSM-5 = 30 (5.2%) < Mo₂C/HZSM-5 = 23 (8.0%).     

Porous Molybdenum Carbide Nanostructured Catalyst toward Highly Sensitive Biomimetic Sensing of H2O2

There are great challenges to fabricate a highly selective and sensitive enzyme‐free biomimetic sensor. Herein for the first time a unique nanostructure of porous molybdenum carbide impregnated in N‐doped carbon (p‐Mo₂C/NC) is synthesized by using SiO₂ nanocrystals‐templating method and is further used as an enzyme‐free electrochemical biosensor toward highly selective, sensitive detection of H₂O₂, of which the limit of detection, dynamic detection range and sensitivity accomplish as 0.22 μM, 0.05–4.5 mM and 577.14 μA mM^?1 cm^?2, respectively, and are much superior to the non‐porous molybdenum carbide impregnated in N‐doped carbon (Mo₂C/NC). The sensor is also used to monitor H₂O₂ released from A549 living cells. This work holds a great promise to be used to monitor the presence of H₂O₂ in biological research while offering an important knowledge to design a highly selective and sensitive biomimetic sensor by synthesizing a porous catalyst to greatly improve the reaction surface area rather than conventionally only relying on dispersing the catalyst material into porous carbon substrate.     

Compositional and structural characterization of alloyed carbide by 3D atom probe and high‐resolution TEM

During tempering of solute supersaturated ferrous martensite, the face‐centered cubic MC‐type carbides (M is alloy elements) such as VC and NbC phases usually co‐precipitate on crystal defects such as dislocation and take on plate‐like morphology. Over‐tempering makes the plate‐like shape change to spherical shape because of Ostwald coarsening. The coarsening process strongly correlates to the diffusion behaviors of the carbon and carbide‐forming elements, and consequently inhomogeneous compositional and structural distribution in the carbides is formed. Three‐dimensional atom probe and high‐resolution transmission electron microscopy have been proved useful methods to characterize the composition, morphology and nanostructure of the carbides that precipitate in a quench‐tempered micro‐alloyed steel. Depending on the actual affinity with C and the diffusion behavior, Si and Al are rejected from the alloy carbide, whereas Mn, V and Nb are inhomogeneously enriched in it. The morphology and structure change with the compositional redistribution. During the coarsening process of the pre‐existing plate‐like carbide, transition carbide that is semi‐coherent with ferritic matrix is formed because of the disparity in diffusion ratio of different solutes. A core–shell complex nanostructure is consequently formed in the coarsening carbide, and the core and shell are identified as V₈C₇ enriched in Mn, Mo and Mo₂C, respectively. Copyright © 2013 John Wiley & Sons, Ltd.     

A Molybdenum Carbide Nanotubes Modified Electrode as the Functionalized Sensing Platform for Electrochemical Detection of Dopamine

In this work, molybdenum carbide (Mo₂C) nanotubes were prepared via the self‐degradable template method and high‐temperature calcination, and then were successfully used to modify glassy carbon electrode (CGE) for the high‐sensitivity determination of dopamine (DA) without the inference of ascorbic acid(AA) and uric acid (UA). The surface morphology of Mo₂C nanotubes has been investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). Owing to the enhanced electron transfer rate and high surface area of Mo₂C, the modified electrode not only exhibits very excellent electrochemical performance for DA, but also has good analytical performance for DA in the mixture with AA and UA through differential pulse voltammetry (DPV), which can be applied for DA detection with wide linear range(0.005–50 μ mol L^?1) and low detection limit 0.001 μ mol L^?1. The modified electrode has been applied to detect DA in DA hydrochloride injection by using standard adding method with satisfactory results.     

Electrochemical and physical characterization of Pt–Ru alloy catalyst deposited onto microporous–mesoporous carbon support derived from Mo2C at 600 °C

The electrochemical reduction of oxygen on binary Pt–Ru alloy deposited onto microporous–mesoporous carbon support was studied in 0.5 M H₂SO₄ solution using cyclic voltammetry, rotating disk electrode (RDE), and impedance method. The microporous–mesoporous carbon support C(Mo₂C) with specific surface area of 1,990 m²?g^?1 was prepared from Mo₂C at 600 °C using the chlorination method. Analysis of X-ray diffraction, photoelectron spectroscopy, and high-resolution transmission electron microscopy data confirms that the Pt–Ru alloy has been formed and the atomic fraction of Ru in the alloy was ～0.5. High cathodic oxygen reduction current densities (?160 A?m^?2 at 3,000 rev?min^?1) have been measured by the RDE method. The O₂ diffusion constant (1.9?±?0.3?×?10^?5?cm²?s^?1) and the number of electrons transferred per electroreduction of one O₂ molecule (～4), calculated from the Levich and Koutecky–Levich plots, are in agreement with literature data. Similarly to the Ru/RuO₂ system in H₂SO₄ aqueous solution, nearly capacitive behavior was observed from impedance data at very low ac frequencies, explained by slow electrical double-layer formation limited by the adsorption of reaction intermediates and products into microporous–mesoporous Pt–Ru–C(Mo₂C) catalyst. All results obtained for C(Mo₂C) and Pt–Ru–C(Mo₂C) electrodes have been compared with corresponding data for commercial carbon VULCAN® XC72 (C(Vulcan)) and Pt–Ru–C(Vulcan) electrodes processed and measured in the same experimental conditions. Higher activity for C(Mo₂C) and Pt–Ru–C(Mo₂C) has been demonstrated.     

Coupling Mo2C with Nitrogen‐Rich Nanocarbon Leads to Efficient Hydrogen‐Evolution Electrocatalytic Sites

In our efforts to obtain electrocatalysts with improved activity for water splitting, meticulous design and synthesis of the active sites of the electrocatalysts and deciphering how exactly they catalyze the reaction are vitally necessary. Herein, we report a one‐step facile synthesis of a novel precious‐metal‐free hydrogen‐evolution nanoelectrocatalyst, dubbed Mo₂C@NC that is composed of ultrasmall molybdenum carbide (Mo₂C) nanoparticles embedded within nitrogen‐rich carbon (NC) nanolayers. The Mo₂C@NC hybrid nanoelectrocatalyst shows remarkable catalytic activity, has great durability, and gives about 100 % Faradaic yield toward the hydrogen‐evolution reaction (HER) over a wide pH range (pH 0–14). Theoretical calculations show that the Mo₂C and N dopants in the material synergistically co‐activate adjacent C atoms on the carbon nanolayers, creating superactive nonmetallic catalytic sites for HER that are more active than those in the constituents.     

Rough-surfaced molybdenum carbide nanobeads grown on graphene-coated carbon nanofibers membrane as free-standing hydrogen evolution reaction electrocatalyst

A novel free-standing pie-like paper electrode composed of Mo₂C nanobeads on graphene-coated carbon nanofibers (G-CNF) membrane was rationally designed as advanced electrocatalyst for hydrogen evolution reaction (HER). A thin layer of graphene is coated on the surface of CNF membrane, forming a “crust” on fibrous web architecture. The unique design of the all-carbon membrane, which is a 3D interconnected conductive framework of nanofibers, reduces the resistance of electron and ion transport during the electrocatalyzing process. With G-CNF performing as support, well-shaped Mo₂C nanobeads were immobilized on the fibers through hydrothermal and calcination procedures, offering rich catalytic sites on the exposed rough surface. Owing to all these merits, the composite membrane of Mo₂C-G-CNF exhibits high HER catalytic activity with onset potential of 115 mV in acidic solution and 108 mV in basic solution. Furthermore, the good durability in both acidic and basic environment guarantees its practical application as free-standing electrode material.     

Effects of Mo2C loading and H2S concentration on Mo2C/Al2O3 catalyst applied in sulfur‐resistant methanation

Mo₂C/Al₂O₃ catalyst was prepared by the impregnation method with urotropine and ammonium paramolybdate. The catalytic effect of Mo₂C as a typical transition‐metal carbide in sulfur‐resistant methanation was studied. The catalysts prepared were characterized by N₂ adsorption–desorption, X‐ray diffraction, transmission electron microscopy, H₂‐temperature‐programmed reduction, and Raman spectra, with the results confirming the formation of β‐molybdenum carbide on the surface of the catalysts. Studies on catalysts with different loading doses indicate that the optimal loading of Mo₂C/Al₂O₃ is about 15 wt.%, which enables CO conversion rate of up to 47%, with methane selectivity of up to 53%. This work further explored the effect of different concentrations of H₂S in the raw gas on the performance of the catalyst, with the results showing that high concentration of H₂S (>1500 ppm) can lead to sulfuration of active species on the catalyst, while resulting in a decrease in the catalytic activity.     

Synergistic Ternary Composite (Carbon/Fe3O4@Graphene) with Hollow Microspherical and Robust Structure for Li‐Ion Storage

The electrode materials with hollow structure and/or graphene coating are expected to exhibit outstanding electrochemical performances in energy‐storage systems. 2D graphene‐wrapped hollow C/Fe₃O₄ microspheres are rationally designed and fabricated by a novel facile and scalable strategy. The core@double‐shell structure SPS@FeOOH@GO (SPS: sulfonated polystyrene, GO: graphene oxide) microspheres are first prepared through a simple one‐pot approach and then transformed into C/Fe₃O₄@G (G: graphene) after calcination at 500 °C in Ar. During calcination, the Kirkendall effect resulting from the diffusion/reaction of SPS‐derived carbon and FeOOH leads to the formation of hollow structure carbon with Fe₃O₄ nanoparticles embedded in it. In the rationally constructed architecture of C/Fe₃O₄@G, the strongly coupled C/Fe₃O₄ hollow microspheres are further anchored onto 2D graphene networks, achieving a strong synergetic effect between carbon, Fe₃O₄, and graphene. As an anode material of Li‐ion batteries (LIBs), C/Fe₃O₄@G manifests a high reversible capacity, excellent rate behavior, and outstanding long‐term cycling performance (1208 mAh g^?1 after 200 cycles at 100 mA g^?1).     

Structural transformations of CuMoO4 in the catalytic oxidation of carbon

The catalytic combustion of carbon black at 350–420°C in the presence of CuMoO₄ has been investigated. The separate catalyst reduction and reoxidation stages make nonadditive contributions to the overall heat of the process. This indicates the formation of catalytically intermediate compounds during the redox reactions. The reduction of the catalyst with carbon yields the copper(I) molybdates Cu₆Mo₅O₁₈ and Cu₄Mo₅O₁₇ on its surface. The reoxidation of the reduced phases is accompanied by the release of Cu₂O and MoO₃ followed by the formation of the active phase Cu_{4 ? x} Mo₃O₁₂, which is capable of activating carbon black combustion.     

Ultrafine molybdenum carbide nanoparticles supported on nitrogen doped carbon nanosheets for hydrogen evolution reaction

Nitrogen doped carbon nanosheets supported molybdenum carbides nanoparticles (Mo_xC/NCS) have been synthesized by tuning the mass ratio of melamine and ammonia molybdate. The Mo₂C/NCS-10 exhibits superior electrocatalytic performance and stability for HER, which was attributed to N-doped carbon nanosheets, small particle size, mesoporous structure, and large electrochemical active surface area.     

The Structure and Stability of Magic Carbon Clusters Observed in Graphene Chemical Vapor Deposition Growth on Ru(0001) and Rh(111) Surfaces

To improve the atomically controlled growth of graphene by chemical vapor deposition (CVD), understanding the evolution from various carbon species to a graphene nucleus on various catalyst surfaces is essential. Experimentally, an ultrastable carbon cluster on Ru(0001) and Rh(111) surfaces was observed, while its structure and formation process were still under debate. Using ab initio calculations and kinetic analyses, we disclose a specific type of carbon cluster, composed of a C₂₁ core and a few dangling C atoms, which is exceptional stable in a size range from 21 to 27 C atoms. The most stable one of them, an isomer of C₂₄ characterized by three dangling C atoms attached to the C₂₁ core (denoted as C₂₁‐3C), is the most promising candidate of the experimental observation. The ultrastability of C₂₁‐3C originates from both the stable core and the appropriate passivation of the dangling carbon atoms by the catalyst surface.