Core-Shell Metal-Ceramic Microstructures: Mechanism of Hydrothermal Formation and Properties as Catalyst Materials

Unique metal-ceramic composites with core-shell microarchitecture (γ-Al₂O₃@Al and spinel-MeAl₂O₄@Al, Me = Zn, Ni, Co, Mn, and Mg) were obtained by a simple hydrothermal surface oxidation (HTSO) of Al metal particles in an aqueous solution of heterometal ions at elevated temperatures (393-473 K). The reactions afforded self-constructed core-shell microarchitecture with Al core encapsulated by the high-surface-area γ-Al₂O₃ or spinel metal aluminates (MeAl₂O₄) shell with various surface morphologies, compositions, and excellent physicochemical properties. Extensive experimental and theoretical investigation with period 3-6 metal elements (Na⁺, Ca²⁺, Sr²⁺, Ba²⁺, K⁺, Fe³⁺, Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, and Mg²⁺) at various metal concentrations and temperatures revealed that the heterogeneous self-construction of spinel-MeAl₂O₄@Al primarily depends on two intrinsic properties of the additive metal ions: (i) thermodynamic stability constant of the metal hydroxide complex and (ii) size of the metal ion. The spinel-MeAl₂O₄@Al microstructures formed with a limited number of hetero metal ions (Me = Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, and Mg²⁺) with (i) moderate rates of the hydroxide formation with compatible kinetics to the hydrolysis of aluminum on the Al surface and (ii) small size of additive metal ions enough for diffusion through the shell layer. As heterogeneous catalyst substrates, these metal-ceramic composites delivered markedly enhanced catalytic performance at intensive reaction conditions because of their facile heat transfer and superior physicochemical surface properties. The performance and effects of the core-shell metal-ceramic composites were demonstrated using Rh catalysts supported on MgAl₂O₄@Al. The Rh/MgAl₂O₄@Al catalyst was utilized for the endothermic glycerol stream reforming (C₃H₈O₃ + 3H₂O ⇄ 3CO₂ + 7H₂, ΔH₀²⁹⁸ = 128 kJ mol^-1), exhibiting markedly greater catalytic performance than the conventional Rh/MgAl₂O₄ under intensive reaction conditions attributed to significantly facilitated heat transport through the core-shell metal-ceramic microstructures.