Heterogeneous flame propagation in the self-propagating, high-temperature, synthesis (SHS) process in multi-layer foils: theory and experimental comparisons

A theory for heterogeneous flame propagation in the self-propagating, high-temperature synthesis (SHS) process that proceeds in multi-layer foils consisting of alternating layers of constituents has been formulated, describing a pre-mixed mode of bulk flame propagation supported by a non-pre-mixed reaction that proceeds at the layer surface of a constituent with higher melting point. The formulation allows for volumetric heat loss throughout the bulk flame, a finite-rate Arrhenius reaction at the layer surface, and temperature-sensitive, Arrhenius mass diffusion in the liquid phase. Results show that the burning velocity is inversely proportional to the layer thickness of the constituent with higher melting point; that the burning velocity decreases with increasing heat loss; that at a critical heat-loss rate, the SHS flame extinguishes, as indicated by the characteristic turning-point behavior; that extinction is sensitively affected by the mixture ratio, initial temperature, layer thickness of the constituent with higher melting point, etc.; that the surface reaction is partly or mainly reaction-controlled when the layer is a few nano-meters thick; that effects of the inter-mixed region produced by inter-facial mixing during deposition can be examined by regarding its components as reaction products that play the role of a diluent; and that the critical layer-thickness that the SHS flame ceases to propagate corresponds to the limit of flammability with respect to dilution. It is further shown that the analytical results agree with available experimental data in the literature, indicating that the present formulation captures the essential features of the non-adiabatic, heterogeneous SHS process that self-propagates in multi-layer foils.     

Facile in-situ formation of high efficiency nanocarbon supported tungsten carbide nanocatalysts for hydrogen evolution reaction

Electrochemical hydrogen evolution reaction (HER) is one of the key techniques for hydrogen production. Much great effort has been made so far to develop highly efficient HER catalysts to replace expensive precious metals (e.g. Pt). Unfortunately, the synthesis processes were generally not cost-effective and/or scalable. So it is highly desirable to develop a facile technique to enhance HER activity of conventional inexpensive but less active materials. In this work, monodispersed tungsten carbide (WC) nanoparticles (<5 nm) were in-situ formed/anchored on nanosized carbon black (CB) and carbon nanotube (CNT) via a simple low temperature molten salt synthesis technique. Owing to this special hybrid structure, both the exposed surface area of active species and the electrical conductivity of the catalysts were increased effectively, making the catalysts perform considerably better in HER than pure WC and WC based catalysts prepared via other conventional routes. WC nanocrystals in-situ formed/anchored on CNTs showed small onset overpotential (90 mV), low Tafel slope (69 mV dec^?1), high current density (93.4 and 28 mA cm^?2 at 200 and 300 mV, respectively) and excellent stability (remaining stable even after 3000 cycles). Such a performance is one of the best among those of WC based electrocatalysts developed to date. We demonstrate here significantly improved HER performances of inexpensive tailored WC materials, along with a facile synthesis strategy which could be also readily extended to prepare a range of other types of mono-dispersed nanocatalysts for more potential applications.     

Facile one-step synthesis of nitrogen-doped carbon sheets supported tungsten carbide nanoparticles electrocatalyst for hydrogen evolution reaction

Exploring non-precious metal catalysts with high activity and stability to replace Pt-based materials is vital for electrochemical water splitting. In this work, a facile one-step method was put forward to synthesize WC/NC composite. Due to the couple effect of KCl/NaCl salt and dicyandiamide, pure WC phase was obtained at 900 °C. Meanwhile, KCl/NaCl salt eliminated the runaway pyrolysis expansion of glucose. Besides contributing to the special surface area, dicyandiamide as N source significantly alleviated W mass loss trigged by KCl/NaCl salt and ensured the appropriate WC content in WC/NC composite. As a HER electrocatalyst in acid media, WC/NC composite exhibits the small overpotential of 156 mV at a current density of 10 mA cm⁻², the low Tafel slope of 64 mV dec⁻¹, as well as the robust stability. This work offers a feasible option to fabricate low-cost and effective transition metal carbide electrocatalysts on a large-scale for hydrogen evolution reaction.     

A numerical performance study of a fixed-bed reactor for methanol synthesis by CO2 hydrogenation

Synthetic fuels are needed to replace their fossil counterparts for clean transport. Presently, their production is still inefficient and costly. To enhance the process of methanol production from CO₂ and H₂ and reduce its cost, a particle-resolved numerical simulation tool is presented. A global surface reaction model based on the Langmuir-Hinshelwood-Hougen-Watson kinetics is utilized. The approach is first validated against standard benchmark problems for non-reacting and reacting cases. Next, the method is applied to study the performance of methanol production in a 2D fixed-bed reactor under a range of parameters. It is found that methanol yield enhances with pressure, catalyst loading, reactant ratio, and packing density. The yield diminishes with temperature at adiabatic conditions, while it shows non-monotonic change for the studied isothermal cases. Overall, the staggered and the random catalyst configurations are found to outperform the in-line system.     

A wide range modeling study of formation and nitrogen chemistry in hydrogen combustion

The chemistry of nitrogen species and the formation of NO_x in hydrogen combustion are analyzed here on the basis of a large set of experimental measurements.The detailed kinetic scheme of H₂/O₂ combustion was updated and upgraded using new kinetic and thermodynamic measurements, and was validated over a wide range of temperatures, pressures and equivalence ratios. The mechanism's performance at high pressures was greatly improved in particular by adopting higher rate parameters for the H+OH+M=H₂O reaction.The NO_x sub-mechanism was further validated and updated. The kinetic parameters of the NO₂+H₂=HONO+H and N₂H₂+NO=N₂O+NH₂ reactions were updated in order to improve model predictions in specific conditions.Sensitivity analyses were carried out to determine which reactions dominate the H₂/O₂ and H₂/NO_x systems at particular operating conditions.Good overall agreement was observed between the model and the wide range of experiments simulated.     

Molten salt synthesis of high-entropy alloy AlCoCrFeNiV nanoparticles for the catalytic hydrogenation of p-nitrophenol by NaBH4

High-entropy alloy (HEA) AlCoCrFeNiV nanoparticles were prepared from oxide precursors using a molten salt synthesis method without an electrical supply. The oxide precursor was directly reduced by CaH₂ reducing agent in molten LiCl at 600°C-700°C or molten LiCl–CaCl₂ at 500°C-550°C. When the reduction was conducted at 700°C, a face-centered cubic (FCC) structure produced, as identified by X-ray diffraction analysis. With lower reduction temperatures, the FCC structure was absent, replaced by a body-centered cubic (BCC) structure. With a reduction temperature of 550°C, the resulting sample was composed of highly pure HEA AlCoCrFeNiV nanoparticles with a BCC structure of 15 nm. Analyses by scanning electron microscopy/transmission electron microscopy with energy-dispersive X-ray spectroscopy confirmed the formation of homogeneous HEA AlCoCrFeNiV with a nanoscale morphology. In the hydrogenation reaction of p-nitrophenol by NaBH₄, the AlCoCrFeNiV nanoparticles (produced at 550°C) exhibited a catalytic activity with ～90% conversion and 16 kJ/mol activation energy.