Nanoconfinement-engineered iron-based redox catalysts: Precise shell thickness gradients enhanced durability of chemical looping hydrogen production

Abstract

Hydrogen energy, as the ultimate clean energy, effectively avoids the greenhouse effect. Chemical looping hydrogen production (CLHP), a versatile energy conversion and production technology, has garnered extensive attention. CLHP demands redox catalysts with high oxygen capacity, regulatable reactivity, and structural integrity even under harsh operational conditions. Currently, sintering, agglomeration, and inactivation of redox catalysts during cyclic lattice oxygen release and restoration are challenging, hindering the wide industrialization of the chemical looping (CL) process. Moreover, the precise control of activity and reaction rate of the redox catalysts to flexibly accommodate the demands of various reaction substrates remains unclear. This paper introduces the design of a nano-scaled redox catalyst featuring a unique core-shell structure. By precisely controlling the shell thickness, a series of hierarchical Fe₂O₃@SiO₂ redox catalysts were successfully synthesized. Building on this achievement, an in-depth investigation was conducted into the impact of the thickness and spatial structure of the inert support on the stability and mass transfer rate of the redox catalyst, aiming to achieve a perfect balance between these two factors during the CLHP process. A thin shell (70 nm) exhibits excellent cyclic stability, maintaining consistent performance in 30 consecutive redox cycles, while a thicker shell (200 nm) undergoes rapid deactivation due to the formation of a substantial amount of iron silicate. In-situ transmission electron microscopy (TEM) reveals that the SiO₂ shell effectively restricts the agglomeration of Fe₂O₃. The unique core-shell structure and controllable shell thickness offer novel insights into the flexible design of efficient and durable hierarchical redox catalysts with spatial structure.