Synthesis and Properties of Al₂O₃@Al Metal-Ceramic Core-Shell Microstructures for Catalyst Applications

Core-shell Al₂O₃@Al microstructures consisting of a highly heat conductive Al metal core encapsulated by a dense γ-Al₂O₃ shell formed by aggregation of porous γ-Al₂O₃ crystallites were obtained by hydrothermal surface oxidation of Al metal particles using two different methods: hydrothermal reactions at elevated temperatures above 423 K under autogenous pressure and microwave-powered surface-activated hydrothermal reactions at atmospheric pressure. The phase transformation of Al into γ-Al₂O₃ at the core/shell interface and the resulting morphological and structural properties of γ-Al₂O₃ crystallites from these two synthesis routes differed significantly. The high temperature hydrothermal route led to formation of densely agglomerated plate and rhombic shaped γ-Al₂O₃ crystallites with properties attributed to temperature, pH, and the presence of anions (NO₃ ^-, Cl^-, SO₄ ^2-) and metal cations (Na⁺, K⁺, Ca²⁺). The microwave-powered method was highly efficient for structure formation under benign temperature and pressure conditions, resulting in uniform core-shell microstructures with unique petal-like surface morphologies and a sharp pore size distribution. These core-shell structured Al₂O₃@Al metal-ceramic composites utilized as supports for Rh catalysts enabled facilitated heat transport for endothermic glycerol steam reforming reactions, which resulted in substantial rate enhancements compared to a conventional Rh/Al₂O₃ catalyst.