Towards understanding the TiO2-mediated photoredox process of Cu(II)-formic acid solution

The performance of TiO2-film was investigated in the process of simultaneous photocatalytic Cu(II) reduction and COD (chemical oxygen demand) removal from Cu(II)-containing formic acid solution during successive batch runs. It was found that, during the first batch run, the Cu(II) ions could increase the COD removal efficiency, synchronous to the Cu(0) deposition, however, decrease the efficiency by a factor of 22.5% with the continuative proceeding of the deposition for the second batch run. Unexpectedly, after the second run the COD removal efficiency remained a constant, ca. 40%, and no longer continued decrease. The unexpected observation was attributed to the cycle of Cu(II) deposition and Cu(0) re-dissolution due to air oxidation at pH 2.73. The attribution was primarily confirmed by the scanning electron microscope image and several fluctuation phenomena in the reaction process, such as pH, Cu(II) concentration and photocurrent.