SYNTHESIS OF EFFICIENT PHOTOCATALYSTS IN EXTREME TEMPERATURE CONDITIONS FOR THE DEGRADATION OF AIR POLLUTANTS.

This research arises from the significant thermal limitations of existing nitrogen oxides (NOx) reduction materials, particularly TiO2, which undergoes an irreversible phase transition (from anatase to rutile) at 600°C, severely compromising its photocatalytic efficiency. In addition, TiO2’s wide energy bandgap (~ 3.2 eV), and a rapid recombination rate of electron-hole pairs hinder its light absorption and overall photocatalytic performance. To overcome these challenges, this study proposes an innovative design strategy for strontium titanate (STO), a material with a similar energy band gap to TiO2 but offering enhanced durability due to its unalterable cubic crystal structure at high temperatures. Therefore, STO enables the development of more efficient systems for NOx photocatalytic reduction under LED irradiation and supports high-temperature industrial applications. This work explored various synthesis methods for the fabrication of STO-based materials. A comprehensive life cycle assessment (LCA) was conducted to assess the sustainability of six different synthesis routes. Preliminary findings indicate that methods with the calcination step as the final stage, such as sol-gel, solid-state, and sonochemical (in ascending order of sustainability), emerged as the most eco-friendly approaches for STO fabrication. These insights provided valuable guidance for the continued development of sustainable, novel, and efficient STO-based photocatalysts in this investigation. Furthermore, modification strategies to enhance the photocatalytic performance of STO, such as the incorporation of noble metal nanoparticles onto the STO surface, were also investigated. The localized surface plasmon resonance (LSPR) effect, along with the formation of the Schottky barrier between the metal and semiconductor, improves electron/hole pair separation, significantly benefiting the overall photocatalytic efficiency of STO-based materials. Among the noble metals, silver (Ag) offers the best balance between cost-effectiveness, visible light photo-response enhancement, and antibacterial properties. Due to these advantages, silver has been widely employed as a TiO2 modifier in literature and is similarly employed in this work to enhance STO performance. Therefore, an Ag-STO system was synthesized through a wet impregnation method, where the STO surface was modified with Ag particles pre-synthesized via the electrochemical method, using Ag+ enriched-solution wastewater as a starting material. This multistep approach led to a significant increase in NOx photodegradation rates, achieving approximately 77% ± 3%. Meanwhile, a novel one-pot, water-based synthesis using AgNO3 as a pure Ag precursor enabled dual modification of STO, incorporating both Ag+ doping into STO crystal lattice and Ag nanoparticles onto STO surface. This dual modification system demonstrated thoroughly NOx degradation under LED illumination within 180 minutes. Moreover, the role of Ag in enhancing electron-hole separation was confirmed, with Ag nanoparticles on the STO surface proving crucial to the photocatalytic efficiency of the system. Lastly, the optimization of solvents, amount of silver, and calcination conditions (temperature and residence time) was evaluated for producing Ag-modified STO photocatalysts. Notably, an 8 wt.% Ag-modified STO sample synthesized via sol-gel, using a 1:3 water/ethanol solvent mixture, exhibited the highest photocatalytic efficiency. And, it was observed that, even with short residence times at 1100°C, the photocatalytic performance did not significantly decrease, demonstrating strong thermal resistance. Overall, this work underscores the development and assessment of novel strontium titanate (STO) - based photocatalysts as effective materials for air purification, offering high thermal stability and sustainability. These findings pave the way for the industrial integration of photocatalysts in air quality management, contributing to environmental sustainability.