Porous Transition Metal Oxides As Robust Catalysts For Water Oxidation

Nowadays,increasing energy consumption and environmental problem require new energy resources to substitute for fossil fuels.Hydrogen fuel generated by water splitting becomes one of the most efficient chemical methods for renewable energy storage.However,the oxygen evolution reaction?OER?remains the bottleneck of water splitting because of the intrinsically very sluggish kinetics associated with multistep proton-coupled electron transfer.Therefore,developing highly efficient water oxidation catalysts is crucial.The iridium and ruthenium oxides represent excellent catalysts for water oxidation reaction,but the scarcity and high-cost of precious metals limit their practical application on a large scale.It is highly desirable and urgent to develop new ocatlysts with both excellent performance and low cost.In this thesis,we focus on studying porous transition metal oxides containing Co,Ni,Fe and Mn as robust catalysts for water oxidation.1.We report mesoporous Mn_1.8Fe_1.2O₄ nanocubes that are synthesized through simple self-assembly and calcination.By changing the calcination temperature,we investigated the correlation between the nanostructure and catalytic performance of mesoporous Mn_1.8Fe_1.2O₄ catalysts.Mesoporous Mn_1.8Fe_1.2O₄ exhibited high oxygen evolution activity in both the photochemical and cerium?IV?-driven water oxidation systems.The highest initial turnover frequency?TOF?of 4.6×10-4s^-1 per metal atom is obtained at neutral pH in photocatalytic water oxidation reaction,compared with those nonporous MnFe₂O₄,Fe₃O₄,and Mn₃O₄.In a cerium?IV?-driven environment,a high TOF of 1.8×10-3s^-1 per metal atom is achieved,which is at least two orders of magnitude more active than that of nonporous MnFe₂O₄.Multiple experimental results?e.g.,XRD,FT-IR,SEM,TEM,and XPS?confirmed that mesoporous Mn_1.8Fe_1.2O₄ materials are highly stable.Our results provide a facile method for synthesis of an inexpensive and highly active heterogeneous catalyst for the oxygen evolution reaction.2.Iron-based catalysts are of particular interest for water oxidation because of its high abundance,low toxicity and rich redox properties.Herein,we report low cost porous iron-based oxides derived from calcining precursors of Prussian blue analogue?PBA?Mx[Fe?CN?6]y?M = Fe,Co,Ni?.This synthesis approach involves a simple self-assemble technology and a low temperature annealing procedure.These catalysts were investigated for photocatalytic,cerium?IV?-driven and electrochemical water oxidation,and they exhibited superior activity.It is noteworthy that this photocatalytic water oxidation was conducted under neutral conditions that are similar to the natural photosystem II.The high initial turnover frequency?TOF?of ?5.4×10-4 s^-1 per transition metal atom at the first 60 seconds is obtained under neutral pH using porous Co_xFe_3-xO₄ in photocatalytic water oxidation reaction,which is comparable with those published iron-based catalysts.Under cerium?IV?-driven water oxidation condition,the TOF of porous Co_xFe_3-xO₄ is 5.2×10-4 s^-1 per transition metal atom,which is the highest value among all the documented iron oxides.In the electrochemical water oxidation,the porous NixFe3-xO4 catalyst exhibits a low overpotential of 402 mV at 10 mA cm-2.Meanwhile,the porous iron-based oxides possess beneficial ferromagnetic properties and excellent stability so that they were used repeatedly without the loss in activity.3.Porous materials are favorable for catalysis because of their high surface areas and rich edge sites.Herein,we report a general strategy to synthesize porous nanosheets of NiMn oxides by calcination of the layered double hydroxide precursor.The porous Ni0.75Mn0.25 nanosheets are robust for electrocatalytic oxygen evolution reaction?OER?with low overpotential 358 mV and high turnover frequency?TOF?of 0.08 s^-1 at ? = 400 mV.By using X-ray absorption spectroscopy?XAS?analysis,we find that the high intrinsic catalytic activity of Ni0.75Mn0.25 due to longer Mn-O-Mn bonds contribute to structural flexibility in the surface.Our work presents a simple doping method to prepare porous nanosheets and to further enhance their catalytic performance by doping other transition element.