Macroaggregation effect of TiO2 nanoparticles on the photocatalytic activity and post-reaction separation for aqueous degradation of organic compounds

Abstract

Photocatalysis for the decomposition of contaminants in water is an excellent purification method. Although nano-TiO2 suspensions provide high activity, solid-liquid separation after the reaction is rather difficult. On the other hand, it has been noted that the activity of immobilized photocatalysts decreases significantly compared to powder-suspended photocatalysts. In this study, macroaggregated granular photocatalysts (MGPs) with secondary particle sizes (40–125 µm) were prepared by dehydrative condensation of OH– groups on the surface of nano-TiO2 under acidic conditions. Powder-free granules were obtained after removing remaining particles in water by sedimentation. Scanning electron microscopy was conducted and the particle size distribution was measured in water by laser diffraction. Particles less than 40 µm in diameter were almost not present in the granules. Relative diffuse reflectance and photoluminescence measurements demonstrated that Pt-loaded MGPs absorbed light more efficiently than the powdered photocatalyst and resulted in less recombination of the separated charges. Pt-loaded MGPs were tested for the decomposition of acetic acid in water under disperse conditions by varying the photocatalyst mass concentration (Cm). The activity of MGPs at Cm in the range of 3–10 g L−1 was higher than that of the photocatalyst dispersed in colloidal suspensions, resulting in a maximum CO2 production rate 1.6 times higher. The activity results of the granular and powdered catalysts, and tests with Pt-loading or no loading, indicated an effective light absorption and electron transfer to Pt via the macroaggregate structure. After the stirring stopped, MGPs completely settled within 1 min, and the recovered photocatalyst was tested again showing approximately 95% of the initial photocatalytic activity. MGPs showed excellent photocatalytic activity and facile solid-liquid separation.