Synthesis Of Transition Metal Carbides Research Articles

Predictive Synthesis of Transition Metal Carbide via Thermochemical Oxocarbon Equilibrium.

Fabricating nanoscale metal carbides is a great challenge due to them having higher Gibbs free energy of formation (ΔG°) values than other metal compounds; additionally, these carbides have harsh calcination conditions, in which metal oxidation is preferred in the atmosphere. Herein, we report oxocarbon-mediated calcination for the predictive synthesis of nanoscale metal carbides. The thermochemical oxocarbon equilibrium of CO-CO2 reactions was utilized to control the selective redox reactions in multiatomic systems of Mo-C-O, contributing to the phase-forming and structuring of Mo compounds. By harnessing the thermodynamically predicted processing window, we controlled a wide range of Mo phases (MoO2, α-MoC1-x, and β-Mo2C) and nanostructures (nanoparticle, spike, stain, and core/shell) in the Mo compounds/C nanofibers. By inducing simultaneous reactions of C-O (selective C combustion) and Mo-C (Mo carbide formation) in the nanofibers, Mo diffusion was controlled in C nanofibers, acting as a template for the nucleation and growth of Mo carbides and resulting in precise control of the phases and structures of Mo compounds. The formation mechanism of nanostructured Mo carbides was elucidated according to the CO fractions of CO-CO2 calcination. Moreover, tungsten (W) and niobium (Nb) carbides/C nanofibers have been successfully synthesized by CO-CO2 calcination. We constructed the thermodynamic map for the predictive synthesis of transition metal carbides to provide universal guideline via thermochemical oxocarbon equilibrium. We revealed that our thermochemical oxocarbon-mediated gas-solid reaction enabled the structure and phase control of nanoscale transition metal compounds to optimize the material-property relationship accordingly.

Synthesis of transition metal carbides and high-entropy carbide TiZrNbHfTaC5 in self-shielding DC arc discharge plasma

The paper shows the feasibility of synthesizing micro- and nano-sized particles of binary metal carbides (Me–C) and high-entropy carbide (HEC) TiZrNbHfTaC5 by vacuum-free electric arc method. The method is based on the effect of self-shielding of the reaction volume from atmospheric oxygen by carbon monoxide CO, which is generated during arcing in air. The synthesis results in a solid solution with a NaCl-type carbide with a cubic lattice, which simultaneously contains atoms of titanium, zirconium, niobium, hafnium, tantalum, and carbon. The lattice parameter of the HEC TiZrNbHfTaC5 phase is ∼4.532 Å that is in line with the known data on this compound. The synthesis product contains micro-sized particle agglomerates of transition metal carbides. The synthesis products also contain nano-sized particles with a shell-core structure, in which the core can consist of metal carbide (TiC, ZrC, NbC, HfC, TaC) or HEC TiZrNbHfTaC5, and the shell is a graphite phase.

Nanoparticle Size Effects on Phase Stability for Molybdenum and Tungsten Carbides

Transition-metal carbides (TMCs) are important materials for a variety of applications and industrial processes, in part because of their variable crystal structures and surfaces. However, the synthesis of TMCs often proceeds through metastable phases during particle growth, the appearance of which cannot be described by traditional phase diagrams. Here, we use density functional theory calculations and thermodynamic analyses to construct particle size-dependent phase diagrams for Mo and W carbides and reveal the relationships between phase stability and TMC nanoparticle size. We compute size-dependent phase diagrams for a wide range of Mo carbide and W carbide phases, determine predicted crystallization pathways during synthesis, and compare model results with experimental data. We provide insights for the influence of nanoparticle size on TMC nucleation and growth during synthesis and provide a computationally guided road map for navigating the synthesis of target TMC surfaces and phases.

Carbothermal and boron carbide reduction of oxides of some transition metals

The study presents a possible mechanism to produce carbides and diborides of transition metals, such as titanium, vanadium, chromium and zirconium. The carbothermal synthesis of transition metal carbides has defined the direct dependence between the thermodynamic stability of oxides and the temperature range of the reduction onset (the stronger the oxide, the higher the value of the temperature is). It reaches 2000-2100, 1500-1600, 1300-1400 and 2100-2200°C for such carbides as TiC, VC0,88, Cr3C2 and ZrC respectively. The same dependence has not been found for the diborides of these metals. Optimum synthesis temperatures for all these compounds lie in the range of 1600-1700 °C. This viable method to produce transition metal carbides consists in the transfer of vaporous higher and lower oxides. Diborides preparation involves the transfer of oxides and boron vapors onto the surface of the carbon material with the subsequent chemical interaction. In the case of carbide-boron reduction of zirconium oxide in excess of boron carbide, the reaction product will be a composite material (B4C – ZrB2). The ceramics based on this composite possesses high performance properties.

The rapid production of multiple transition metal carbides via microwave combustion under ambient conditions.

Transition metal carbides (TMCs) have attracted great interest owing to their potential applications for energy storage and electrocatalysis. But the synthesis of high-quality TMCs usually needs high energy consumption, long reaction times, or dangerous chemical reagents, limiting their practical application. Here, a microwave combustion method is developed to rapidly (∼2 min) produce transition metal carbides under ambient conditions. The as-synthesized TMCs (W2C, VC, Fe3C, NbC, TaC, and Mo2C) and rGO composites exhibit outstanding catalytic performance for the hydrogen evolution reaction (HER) over a broad range of pH values. This work highlights a novel strategy for the design and synthesis of transition metal carbides.

Transition Metal Carbide Complex Architectures for Energy-Related Applications.

Transition metal carbides (TMCs), as a family of special interstitial alloys, exhibit novel intrinsic characteristics such as high melting point, high electronic conductivity, excellent mechanical and chemical stability, and good corrosion resistance, and hence have attracted ever-growing attention as promising electrode materials for energy-related applications. In this regard, we give a comprehensive overview of the structural design of transition metal carbide complex architectures and their structure advantages for energy-related applications. After a brief classification, we summarize in detail recent progress in controllable design and synthesis of TMCs with complex nanostructures (e.g., zero-, one-, two-, and three-dimensional, and self-supported electrodes) for electrochemical energy storage and conversion applications including metal-ion batteries, supercapacitors, rechargeable metal-air batteries, fuel cells, and water splitting. Finally, we end this review with some potential challenges and research prospects of TMCs as electrode materials for energy-related applications.

Nitrogen-doped graphene–vanadium carbide hybrids as a high-performance oxygen reduction reaction electrocatalyst support in alkaline media

A high-efficiency and stable electrocatalyst for oxygen reduction reaction (ORR) is critical for fuel cells. Here we report a new type of ORR catalyst composed of platinum nanocrystals loaded on nitrogen-doped graphene–vanadium carbide (VC) hybrids. This is the first report on using graphene oxide as the carbon source in the synthesis of transition-metal carbides as ORR catalysts; the electrochemical tests and theoretical modeling prove that the N-doping in both VC and graphene could effectively improve the overall catalytic activity. The catalytic performance of the hybrids in alkaline solutions is superior to that of commercial Pt/C catalysts in terms of the oxygen-reduction half-wave potential and the mass activity. The stability study of the catalyst also shows less degradation in catalytic activity after 3000 cycles compared with Pt/C and the hybrid catalyst structure remains virtually unchanged. Therefore, using inexpensive hybrids of high-conductivity nitrogen-doped transition-metal carbides and nanocarbon as the catalyst support presents a new direction to optimize catalyst performance for next-generation fuel cells.

Combustion synthesis of metal carbides: Part I. Model development

The definition of a rigorous theoretical framework for the appropriate physico-chemical description of self-propagating high-temperature synthesis (SHS) processes represents the main goal of this work which is presented in two sequential articles. In this article, a novel mathematical model to simulate SHS processes is proposed. By adopting a heterogeneous approach for the description of mass transfer phenomena, the model is based on appropriate mass and energy conservation equations for each phase present during the system evolution. In particular, it takes microstructural evolution into account using suitable population balances and properly evaluating the differentdriving forces from the relevant phase diagram. The occurrence of phase transitionsis treated on the basis of the so-called enthalpy approach, while a conventional nucleation-and-growth mechanistic scenario is adopted to describe quantitatively the formation of reaction products. The proposed mathematical model may be applied to the case of combustion synthesis processes involving a low melting point reactant and a refractory one, as for the synthesis of transition metal carbides from pure metal and graphite. Thus, the model can be profitably used to gain a deeper insight into the microscopic elementary phenomena involved in combustion synthesis processes through a suitable combination of experimental and modeling investigations, as it may be seen in Part II of this work [J. Mater. Res. 20, 1269 (2005)].

Group VI transition metal carbides as alternatives in the hydrodechlorination of chlorofluorocarbons

The synthesis of transition metal carbides of tungsten and molybdenum has been carried out via temperature programmed reactions (TPRs) of metal oxides or passivated nitrides. Their specific chloropentafluoroethane conversion rates were at best one order of magnitude less than that of a reference Pd based catalyst. The intrinsic rates range from 4.7 to 14.7 nmol m −2 s −1 and decrease as follows: Mo 2C>WC>W 2C≈WC 1− x >MoC 1− x . The group VI carbide samples catalyse hydrodehalogenation and dehydrofluorination. WC appears to be as selective towards pentafluoroethane (HFC-125) as the Pd based catalyst. Then the selectivity decreases in the following sequence: W 2C>Mo 2C>WC 1− x >MoC 1− x . All the carbide catalysts deactivate at the early stages of the reaction. Based on the XPS results and the product distribution of the reaction, the deactivation has been mainly attributed to a site blocking phenomenon due to a strong deposit of polymeric carbon and of hydrofluorocarbon polymers. Polymerisation of detected unsaturated compounds take place on acidic sites probably generated by fluoride and/or chloride in the course of the reaction.