Enhanced catalytic ozonation performance by CuO nanoclusters/TiO nanotube and an insight into the catalytic mechanism.

Highly reactive nanoclusters of metal oxides are extremely difficult to be synthesized due to their thermodynamic instability. For the first time, CuO nanoclusters supported on anatase TiO nanotubes (NT) with many defects as anchoring sites were successfully prepared. Although the copper loading reached as high as 2.5 %, the size of CuO nanoclusters in the sample of 2.5 %CuO/NT were mainly around 1.0 nm. The aggregation of copper species during the calcination process was undoubtedly hampered by the anchoring effects of the abundant defects in NT support. Due to the highly exposed undercoordinated atoms of CuO nanoclusters, the mixed valences of copper, and the strong interface interaction between CuO nanoclusters and NT support, 2.5 %CuO/NT-catalyzed ozonation showed the highest pseudo-first-order reaction rate constant of 8.5 × 10 min, 2.2 and 4.0 times that of NT-catalyzed ozonation and ozonation alone, respectively. Finally, the catalytic mechanism was revealed by both experiments and density functional theory calculations (DFT). The results demonstrated that the undercoordinated Cu in CuO/NT could highly promote the adsorption of ozone with a high adsorption energy of -125.16 eV and the adsorbed ozone was activated immediately, tending to dissociate into a O molecule and a surface O atom. Thus, abundant reactive oxygen species, e.g., hydroxyl radical (·OH), superoxide radical (·O) and singlet oxygen (O), could be generated via chain reactions. Especially, ·OH mainly contributed to the removal of ibuprofen pollutants. This work sheds a light on the design and preparation of highly reactive nanoclusters of metal oxide catalysts for catalytic ozonation of refractory organic pollutants.