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.