Shuonan Ye

Electrochemical oxidation significantly lowers the effective interfacial tension of liquid metals in electrolytes, while the resulting oxide layer exhibits a unique combination of fluid- and solid-like characteristics. Here, we uncover the mechanisms underlying these phenomena by isolating the oxide layer via a novel extraction process and probing its microstructure. We find that the oxide layer comprises densely packed nanoparticles, each consisting of a liquid metal core surrounded by an oxide shell. Based on this insight, we propose a nanoparticle jamming mechanism to explain both the dramatic interfacial tension reduction and the fluid-solid duality of the oxide layer. The oxidation-induced volume expansion of these core-shell nanoparticles generates compressive stress within the oxide layer, counterbalancing the intrinsic interfacial tension of the liquid metal and thereby substantially lowering its effective interfacial tension. Concurrently, the jamming transition among nanoparticles imparts the oxide layer with a coexistence of fluidity and solidity. These results reveal a fundamental physical mechanism governing liquid metal interfaces under electrochemical conditions with broad implications for interface engineering and the design of soft functional materials.

The direct reduction of iron ore using hydrogen faces challenges associated with hydrogen storage, transport, and on-site handling. Ammonia (NH₃), with its high hydrogen content, established distribution infrastructure, and economic viability, has emerged as a promising alternative reductant. Here, we employ density functional theory calculations to elucidate the atomic-scale mechanisms governing NH₃ adsorption, dehydrogenation, and nitrogen incorporation on the Fe₃O₄(001) surface. Our results show that NH₃ preferentially adsorbs upright at the surface Fe sites, initiating a sequence of dehydrogenation steps. Among the three dehydrogenation reaction pathways examined, H migration is identified as the rate-determining step for H₂O formation and desorption, a process that generates surface oxygen vacancies. The resulting NH and N species strongly bind to the surface through multiple Fe–N and Fe–NH coordination bonds. Notably, the most favorable configuration─NH binds adjacent to an oxygen vacancy─facilitates further NH dissociation into N and H. The generated vacancies migrate favorably into the subsurface, enabling N incorporation into the lattice and promoting the formation of Fe nitride. Concurrently, N atoms that do not incorporate recombine to form N₂, thereby preventing excessive N accumulation on the surface. These results provide atomistic insights into NH₃-driven Fe₃O₄ reduction and reveal the coupled vacancy dynamics, H mobility, and N incorporation pathways that underpin NH₃-based ironmaking, highlighting the mechanistic opportunities for optimizing sustainable iron ore reduction and advancing NH₃-enabled catalytic processes.