Development of (TAML) iron complexes for photocatalytic, oxidative water treatment

• Main Author: Lim, Jia Hui
• Other Authors: Soo Han Sen
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Science::Chemistry::Physical chemistry::Photochemistry
• Online Access: http://hdl.handle.net/10356/74180
• Institution: Nanyang Technological University

The current energy demand is met mainly by the combustion fossil fuels. This unsustainable energy source brings about negative environmental effects. Using solar energy for water treatment is an appealing solution. To achieve this undertaking, we hypothesized that (TAML)Fe caomplexes, which have been reported to be excellent catalysts for the degradation of persistent pollutants, can be activated photochemically, when coupled with suitable photosensitizers in the right conditions. The project began with the syntheses of the previously reported (TAML)Fe (1), a new (TAML-COOMe)Fe (2), and a new (TAML-COOH)Fe (3). We uncovered a new synthetic route that utilizes nitroaniline precursors instead of o-phenylenediamines, enabling access to a wider range of (TAML) ligand systems. Subsequently, we characterized the electrochemical properties of complexes 1 and 2 using cyclic voltammetry. We have shown that Ru(bpy)32+ should be a suitable photosensitizer to oxidize 1 and 2 photochemically. Hence, we performed transient absorption spectroscopic (TAS) experiments on 1 in a Ru(bpy)32+/Co(CH3)5Cl3 photosensitizer system. The TAS results have verified that the Ru(bpy)32+/Co(NH3)5Cl3 photosensitizer system is able to oxidize 1 by at least one electron, producing a relatively long-lived oxidized 1 intermediate. The CV results also suggested the possibility of accessing a rarely reported three-electron oxidized 2 (formally FeVI complex). Thus we oxidized 2 lectrochemically and chemically at low temperatures. The preliminary results indicate that the three-electron oxidized 2 (23+) may have been generated, and efforts to characterize this intermediate spectroscopically and by X-ray crystallography are still ongoing. However, the oxidation of 1 by the Ru(bpy)32+/Co(NH3)5Cl3 photosensitizer system gave rise to an anion-dependent valence tautomerization of the oxidized 1 intermediate. Through mechanistic studies, kinetics measurements, mass spectrometry, and Mössbauer spectroscopy, we showed that ligand-based oxidation occurs to give 1-TAML•+ in equilibrium with FeIV, instead of the conventional electrophilic halide reactivity. The extent of valence tautomerization depends on the exogenous anions present. A strongly-coordinating CN– will become an axial ligand to stabilize FeIV, and we have isolated and structurally characterized an unprecedented dicyano (TAML)FeIV complex. On the other hand, labile halides favor the FeIII 1-TAML•+ tautomer, which is susceptible to nucleophilic substitution on the ligand. Finally, we anchored 3 on g-C3N4 as a photosensitizer, and characterized the g-C3N4-3 composite using DRS, IR spectroscopic, and XAS measurements. Photocatalytic degradation experiments suggested that 3, after being oxidized by the hole(s) from the VB of g-C3N4, might have recombined faster with the electrons in the CB. To further minimize the effect of charge recombination, we adopted a photoelectrochemical approach and used an electric field instead of a chemical electron acceptor. We also improved the poor interfacial charge transfer from g-C3N4 to the FTO electrode by annealing the g-C3N4 onto FTO glass before doping it with 3. Preliminary evaluations of the electrodes using CV and photoelectrical experiments showed that the doping of the annealed FTO-g-C3N4 electrode with 3 lowered the onset potential and increased the amount of photocurrent. These promising results have propelled us to explore the photoelectrocatalytic degradation of pollutants.