Ultrasound to improve both synthesis and pollutants degradation based on metal nanoparticles supported on TiO2.

Sonochemistry is based on acoustic cavitation, which consist in the formation, growth, and implosive collapse of bubbles within a liquid. Collapsing bubbles generate localized hot spots, characterized by temperatures up to 5000 K and pressures up to 1800 atm. These extreme conditions allow producing a variety of nanostructured and amorphous materials, as well as they are advantageous for chemical processes. Ultrasound requires inexpensive equipment and fewer steps than conventional methods. Combining ultrasound and photocatalysis enhances the performance of the processes, reduces reaction time, avoids the use of extreme physical conditions and improves the photocatalytic materials properties increasing their activity. Here, we reported the positive effect of US in synthesizing Me-modified TiO2 (Me = Ag, Cu, Mn) for pollutants degradation in gas-phase; also, we proved the advantageous application of ultrasound for the photocatalytic removal of organic compounds in water. Ultrasound produced more efficient Me-doped TiO2, which showed higher activity in visible light. When combined with photocatalytic water treatment, the organic compounds degradation and mineralization increases.

Water treatment: Mn-TiO2 synthesized by ultrasound with increased aromatics adsorption.

Pharma-products are mostly single or multiple cyclic compounds. They pollute surface water and are persistent in the aquatic ecosystem. When irradiated by UV light, TiO2 catalysts cleave or degrade organic contaminants in water. Removal of organics by photocatalysis results from a synergistic effect of adsorption and photocatalysis. Synthesis of catalysts by ultrasound (US) produces high surface area and porous solids. Here, we synthesized Mn-doped TiO2 with a US-assisted sol-gel method. Compared to the classical synthesis, US increased the BET surface area from 83 m2 g-1 to 90 m2 g-1 in the Mn-TiO2 sample and from 9.0 m2 g-1 to 53 m2 g-1 in the control TiO2. Accordingly, acetaminophen and amoxicillin adsorption increased from 44% to 52%, and from 34% to 94% for the Mn-TiO2 obtained in absence and presence of US, respectively. When in a mixture, the two drugs strongly compete for adsorption on TiO2.

Ultrasound assisted synthesis of Ag-decorated TiO2 active in visible light.

Titanium dioxide is the most popular photocatalyst to degrade organic pollutants in air, as well as in water. The principal drawback preventing its commercial application lies in its limited absorption of the visible light (400-700nm), while it is active under UV irradiation (≤387nm). Supporting noble metals in the form of nanoparticles on TiO2 increases its activity in the visible range. However, both the synthesis of noble metal nanoparticles and their deposition on TiO2 are multi-step processes that often require organic solvents. Here, we deposit Ag nanoparticles from AgNO3 on the surface of micrometric TiO2 with H2O as a solvent and under ultrasound irradiation at 30Wcm-2. Ultrasound increases the surface amount of Ag on TiO2 with heterogeneous size distribution of Ag nanoparticles, which are bigger and overlaid (1-20nm vs. 0.5-3nm) compared to the sample obtained in traditional conditions (TEM images). While this change in morphology had no effect on acetone photodegradation under UV light, the 5%, 10%, and 20% Ag-TiO2 degraded 17%, 20% and 24% acetone under visible light, respectively. The 10% by weight Ag-TiO2 sample obtained in absence of ultrasound only degraded 14% acetone in 6h, while the bare TiO2 was not active.