Photocatalytic reduction of Hg(II) on TiO2 and Ag/TiO2 prepared by the sol–gel and impregnation methods

Heterogeneous photocatalysis is a convenient tool for mercury(II) reduction, because it uses inexpensive chemicals and near-UV light. However, structural features of titania (band gap at 3.2 eV) make extending its optical absorption in the visible region difficult. This key factor results in low photocatalytic efficiency, as about only 5% of the solar spectrum is used. This study focuses on the characterization and evaluation of the photocatalytic activity of TiO2. The titania preparation method (commercial and sol–gel), the presence of metal on the surface of the oxide (2% wt. Ag), and the calcination temperature (350 and 500 °C) were evaluated. The characterization techniques used were: temperature-programmed reduction (TPR), determination of specific surface area (BET method), X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoacoustic spectroscopy. The photocatalytic reduction of Hg2+ to Hg0 was performed by aqueous solution containing formic acid. The results showed that the presence of Ag on the surface of TiO2 (commercial and sol–gel) decreased the band gap energy and increased the photoactivity efficiency, i.e., all the Hg2+ in the solution was reduced to Hg0.

Research Highlights
► Heterogeneous photocatalysis for mercury(II) reduction.
► Photocatalyst prepared by sol–gel and impregnation method.
► Photocatalytic activity influenced by photocatalyst synthesis method.
► Sol–gel method was photocatalytically more active.