Photocatalytic degradation of formic acid using Fe/TiO2 catalysts: the role of Fe3+/Fe2+ ions in the degradation mechanism

The photocatalytic degradation of formic acid by Fe-doped (0.5, 2 and 5% w/w in Fe) and calcined (773 and 1073 K) TiO2 has been studied. For 773 K calcined catalyst, results suggest that Fe is extracted through the formation of a [FeOOCH]2+ complex by which formic acid degrades. Fe2+ ions that remain in solution after formic acid degradation go back to the catalyst surface and are oxidized by photogenerated holes. In this way the catalyst is reactivated becoming ready for a new degradation process. The formation of hydrogen-carbonates on the TiO2 catalyst through OH radicals insertion has been detected by FTIR. This way the formic acid degradation by OH insertion is inhibited and the formation of [FeOOCH]2+ complex is favored. On the other hand, a markedly lower capacity for the formic acid degradation has been determined for the 1073 K calcined catalysts. This behavior can be attributed to the lower surface area and the major presence of lower active phases such as rutile and Fe2TiO5.