(394c) Nanocrystalline Mesoporous Gallium Oxide Phases

Mesoporous gallium oxide and gallium-based oxides are of great interest in the field of heterogeneous catalysis. Different polymorphs of gallium oxide have been employed for the dehydrogenation of alkanes to alkenes, the epoxidation of alkenes in the presence of hydrogen peroxide, and in the aromatization of ethane in the presence of CO₂. Gallium oxide also exhibits luminescence and conduction properties with potential applications in optoelectronic devices, high-temperature/high-power electronic devices and high-temperature stable gas sensors. Gallium oxide has been synthesized using diverse techniques, such as thermal decomposition, homogeneous precipitation using ammonia, and surface layer adsorption. However these methods offer poor control over structural, morphological and compositional properties.

Surfactant-assisted self-assembly approach reported by researchers at Mobil Research and Development Corporation represents an alternative and attractive method for the synthesis of novel transition metal oxide catalytic phases with desirable structural, compositional and morphological properties. In the present study, we report the synthesis of thermally stable mesoporous gallium oxide employing, Evaporation-Induced Self-Assembly (EISA) and Self-Assembly Hydrothermal-Assisted (SAHA) approaches. These methods eliminate the need for high synthesis temperatures commonly required for solid-state reactions and offer the possibility to synthesize thermally stable mesoporous oxides with controlled morphological, textural and structural properties. Thermally stable semi-crystalline and fully crystalline mesoporous gallium oxide phases were successfully synthesized in the presence of ionic and non-ionic structure directing agents. EISA led to partially crystalline mesoporous gallium oxide phases displaying unimodal pore size distribution in the ~2-5 nm range and surface areas as high as 300 m²/g. SAHA led to nanocrystalline mesoporous uniform micron-sized gallium oxide spheres (~0.3-6.5 µm) with narrow size distribution displaying cubic spinel type structure. These mesophases displayed surface areas as high as 220 m²/g and unimodal pore-size distribution in the 5-15 nm range. Textural properties such as surface areas and pore sizes were effectively fine-tuned by the nature and relative concentration of the structure directing agents.

Keywords: Mesoporous gallium oxide, Thermal stability, Catalysis, Nanocrystals.