Visible-light-driven photocatalytic reduction of Cr(VI) over EDTA-TiO2 through surface complexation and ligand-to-metal charge transfer.

The energy band gap of commercial TiO2 (P25, ~ 3.04 eV) is slightly narrower than that of pure TiO2 (~ 3.2 eV), yet remains inactive under visible light. In contrast, ligand-to-metal charge transfer (LMCT) introduced by surface complexation with organic ligands can significantly enhance light absorption and effectively narrow the band gap. Here, electron-rich ethylenediaminetetraacetic acid (EDTA) serves as an electron donor and coordinates with TiO2 via carboxylate linkers. As a result, the EDTA-TiO2 complex exhibits efficient visible-light-driven photoreduction of toxic Cr(VI) to non-toxic Cr(III). Individually, the optimal loading of EDTA was 200 ppm, which further decreased to 100 ppm upon the introduction of methanol (MeOH, 20 vol%) as a hole scavenger and/or electron donor. Upon visible light illumination, electrons from the HOMO of EDTA move towards the conduction band (CB) of TiO2 through the LMCT process, enabling visible-light absorption and driving the efficient reduction of Cr(VI) to Cr(III). The characteristic FT-IR bands at 1410 and 1677 cm- 1 confirm EDTA-TiO2 complex formation, which is further supported by the calculated adsorption energy (Eads = -18.83 kcal/mol). This study provides mechanistic insight into the adsorption mode of EDTA through carboxylate coordination and elucidates the role of LMCT in enhancing light-harvesting efficiency.