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 3.2eV) 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.