Solar Activation of Molecular Water Oxidation Catalysts Using Semiconductors.

[摘要] Water splitting has been proposed as a viable route toward the renewable production of hydrogen. However, this approach is limited by the water oxidation half-reaction due to the large energy input needed the reaction is sluggish. Sunlight has been used to create excited state electron/hole pairs that are thermodynamically capable of generating H2 and O2. Molecular (homogeneous) and heterogeneous catalysts have been developed to oxidize water efficiently under illumination. The focus of this dissertation is to anchor molecular catalysts onto heterogeneous light absorbers (semiconductors) to more efficiently oxidize water using sunlight. The most significant discovery is that when a molecular Fe water oxidation catalyst is anchored to WO3, a 60% increase in the rate of oxygen evolution and a 40% increase in selectivity towards water oxidation is achieved. Additionally, when different tsemiconductors are used, the reaction rate is dependent on the energy of the conduction band edge and the band gap. For instance, when the conduction band potential energy is held constant and the band gap is decreased from 2.7 eV (WO3) to 2.1 eV (Fe2O3) the rate enhancement with the Fe catalyst increases from 60% to 273%.In addition to the role of the semiconductor on the photoelectrochemical performance, the molecular species also plays an important role in the photoelectrochemical rate enhancement. The most significant development using various molecular species is that replacing iron with other first-row transition metals (Mn-Zn) results in complexes that are only active toward water oxidation when anchored to WO3. And, the corresponding nickel and copper complexes increase the selectivity towards water oxidation up to 99% compared to bare WO3 (56%). Unfortunately, when the best-performing ruthenium catalyst is anchored to various semiconductors, the rate enhancement for solar water oxidation is negligible compared to the other molecular species used in this work. This work demonstrates for the first time that photoelectrochemically generated minority carriers (holes) from semiconductors can be used directly as oxidants to activate molecular oxidation complexes under solar irradiation. Furthermore, this work is the first to quantify the increase in selectivity towards water oxidation directly for WO3 when modified with molecular species.

[发布机构] University of Michigan

[效力级别] Photoelectrochemical Water Splitting

[关键词] Solar Energy;Photoelectrochemical Water Splitting;Semiconductor Photocatalysis;Molecular Photocatalysis;Water Oxidation;Chemistry;Science;Chemistry