Iron-based Oxides For The Photo- And Electrochemical Oxidation Of Water And Organics

The conversion of intermittent solar energy into storable hydrogen resources via water splitting is a promising way. to satisfy the future global energy demand and alleviate the environmental and climate concerns. Water oxidation, however, is the bottleneck among the overall reaction due to its endergonic thermodynamics and complicated 4H+/4e" transfer process. Recognizing the challenge, extensive efforts have been made to develop efficient and robust water oxidation catalysts. In order to realize the largescale manufacturing, the catalysts should be synthesized via facile methods, composed of abundant materials, operated under benign conditions, and easily modified on semiconductor.In this thesis, a highly active iron-based water-oxidation catalyst was electrodeposited from neutral CO2/HCO3- solution containing Fe2+. This catalyst (Fe-Ci) produces a current density of 10 mA/cm'2 at an overpotential of 560 mV in the environmentally benign HCO3-/CO32- buffer (pH 9.75). A stable current density of 4 mA/cm2 sustained for at least 18 h was obtained during constant potential electrolysis at 1.30 vs. NHE without resistance compensation. A Tafel slope of 34 mV/dec was obtained for Fe-Ci, which is significantly lower than other reported values for iron-based catalysts. Kinetic analysis reveals a 2e-/2H+ pre-equilibrium process prior to the chemical rate-determining step. Furthermore, Fe-Ci modified hematite also showed improved photocurrents.An alternative approach to direct water-splitting is coupling proton reduction with photochemical selective oxidation of hydrocarbons. Such reaction has more favorable kinetics and can generate valuable chemicals. Due to the excellent stability of the semiconductor and the high selectivity of the molecular catalysts, molecular ruthenium catalysts modified hematite were employed as the composite catalysts in this thesis. The composite catalyst could catalyze the oxidation of thioanisole with nearly 100% conversion efficiency in the presence of visible light and sacrificial reagents. The composite also showed 5 times higher efficiency than the system with molecular catalysts and hematite particles just dispersed in the case of benzyl alcohol dehydrogenation. A photoelectrochemical cell based on molecular ruthenium catalysts modified hematite photoanode was further fabricated and exhibited improved photocurrent towards the selective oxidation of organics.