Sacrificial Silica Coating-Induced Phase Stability in Mesoporous γ-Alumina

This study investigates the sacrificial role of silica layers in suppressing the phase transformation of γ-alumina (Al₂O₃). Mesoporous γ-Al₂O₃ was coated with silica and subsequently calcined. The silica coating promoted interfacial contact and delayed the γ-to-θ and subsequent α-phase transformations of alumina by approximately 200 °C, as confirmed by X-ray diffraction and porosity analyses. Furthermore, the stabilized γ-Al₂O₃ retains its mesoporous structure after calcination. Solid-state ²⁹Si and ²⁷Al nuclear magnetic resonance (NMR) spectra revealed limited silica polycondensation on mesoporous γ-Al₂O₃, as evidenced by Q² species centered at -90 ppm and their conversion to Q⁰ species around -85 ppm upon calcination. Moreover, the solid-state ²⁷Al NMR spectrum of the coated specimen after calcination at 1100 °C remained similar to that of pristine mesoporous γ-Al₂O₃ showing both 4- and 6-fold Al signals, whereas the calcined uncoated γ-Al₂O₃ exhibited only a 6-fold Al signal associated with the α-phase. These results demonstrate that silica-alumina diffusion occurs while preserving the γ-phase structure through intimate contacts between alumina and the silica coating. This offers a promising strategy for enhancing the thermal stability of as-prepared and unmodified alumina materials. Therefore, the results in this study show the potential of sacrificial silica coatings to stabilize metastable phases and to design nanostructured materials optimized through interfacial diffusion.