Mechanocatalytic Synthesis of Ammonia: State of the Catalyst During Reaction and Deactivation Pathway.

Ammonia synthesis holds significant importance for both agricultural fertilizer production and emerging green energy applications. Here, we present a comprehensive characterization of a catalyst for mechanochemical ammonia synthesis, based on Cs-promoted Fe. The study sheds light on the catalyst's dynamic evolution under reaction conditions and the origin of deactivation. Initially, elemental Cs converts to CsH, followed by partial CsOH formation due to trace oxygen impurities on the surface of the Fe metal and the equipment. Concurrently, the mechanical milling process comminutes Fe, exposing fresh metallic Fe surfaces. This comminution correlates with an induction period observed during ammonia formation. Critical to the study, degradation of active Cs promoter species (CsH and CsNH2) into inactive CsOH emerged as the primary deactivation mechanism. By increasing the Cs content from 2.2 mol % to 4.2 mol %, we achieved stable, continuous ammonia synthesis for nearly 90 hours, showcasing one of the longest-running mechanocatalytic gas phase reactions. Studies of the temperature dependence of the reaction revealed negligible bulk temperature influence in the range of -10 °C to 100 °C, highlighting the dominance of mechanical action over bulk thermal effects. This study offers insights into the complex interplay between mechanical processing, reactive species, and deactivation mechanisms in mechanocatalytic ammonia synthesis.

Reconstruction of Covalent Organic Frameworks by Dynamic Equilibrium.

Covalent organic frameworks (COFs) are periodic two- or three-dimensional polymeric networks with high surface areas, low density, and designed structures. Because COFs are normally prepared based on reversible formation of covalent bonds with relatively weak stability, their structures can be easily broken or damaged due to changes in the surrounding environment. Thus, developing strategies to realize the reconstruction of COFs in order to extend their usage lifetime is crucial for practical applications. In addition, exploring the kinetics of COF growth under varied reaction conditions is important for better understanding the nucleation and growth processes of COFs. In this work, the reformation mechanism of an imine-based COF using an ex situ characterization method was investigated, disclosing an interesting COF reconstruction progress from disorder to order. The present study shows the regeneration ability of COFs, and the developed method could be generalized for broader use in the field.

Copper-doped layered Fe2VO4 nanorods for aqueous zinc-ion batteries.

Zinc-ion batteries (ZIBs) have attracted an increasing attention as a potential low-cost, environmentally friendliness, and high-safety energy storage system. Among them, transition metal vanadates with high oxidation state vanadium have great potential in ZIBs cathode research due to their high theoretical capacity. However, many vanadate particles still inevitably suffer from low ion mobility, low electrical conductivity and stability. Cation doping or compositing is an effective pathway capable of enhancing electrical conductivity. In this work, layered Cu-Fe2VO4 porous nanorods are obtained by introducing Cu2+ into MIL-88A(Fe) (a metal-organic framework; MIL stands for materials from Institute Lavoisier) and further ion-exchanged with NH4VO3, exhibiting excellent zinc storage properties as an cathode. The existence of oxygen vacancies and the change of electronic structure caused by Cu2+ substituting part of Fe2+ enhanced the conductivity and electron transfer rate. It delivers a reversible discharge capacity of 237 mAh/g at 0.3 A/g and a satisfactory high rate capacity of 126 mAh/g after 30 cycles at 5 A/g, and stable cycling performance (198 mAh/g after 1000 cycles at 1 A/g). Furthermore, the energy density can reached to 230.97 Wh kg-1 at 208.6 W kg-1. The assembled quasi-solid-state ZIBs maintain a high capacitance retention of 75% after 8000 cycles at 1 A/g.

An in situ and ex situ study of the microstructural evolution of a novel lithium silicate glass-ceramic during crystallization firing.

OBJECTIVE: To elucidate the compositional and microstructural developments of a novel lithium silicate glass-ceramic during its crystallization cycle. METHODS: Blocks of a lithium silicate glass-ceramic (Obsidian®, Glidewell Laboratories) were cut into 1mm thick plates and polished to 1µm finish. Some of them were crystallized prior to polishing. Firstly, ex situ compositional and microstructural characterizations of both the pre- and post-crystallized samples were performed by wavelength dispersive X-ray fluorescence, field-emission scanning electron microscopy, and X-ray diffractometry. Secondly, the pre-crystallized samples were subjected to in situ compositional and microstructural characterizations under non-isothermal heating by simultaneous thermogravimetry/differential scanning calorimetry, X-ray thermo-diffractometry, and field-emission scanning electron thermo-microscopy. RESULTS: The microstructure of pre-crystallized Obsidian® consists of an abundant population of perlitic-like/dendritic lithium silicate (Li2SiO3) nanocrystals in a glass matrix. Upon heating, the residual glassy matrix does not crystallize into any form of SiO2; elemental oxides do not precipitate unless over-heated above 820°C; and the Li2SiO3 nanocrystals do not react with the glassy matrix to form typical lithium disilicate (Li2Si2O5) crystals. Nonetheless, the Li2SiO3 nanocrystals grow and spheroidize through the solution-reprecipitation process in the softened glass, and new lithium orthophosphate (Li3PO4) nanocrystals precipitate from the glass matrix. SIGNIFICANCE: The identification of compositional and microstructural developments of Obsidian® indicates that, by controlling the firing conditions, it is possible to tailor its microstructure, which in turn could affect its mechanical and optical properties, and ultimately its clinical performance.