| Artificial photosynthesis is a promising strategy for accomplishing the collection and storage of solar energy based on splitting water into oxygen and hydrogen.The splitting of water is a high energy process and can be divided into two steps: the oxidation of water to give oxygen and the reduction of water to produce hydrogen.Water oxidation involving a four-electron transfer is a critically challenging step in water splitting for solar fuel production.Therefore,the development of stable,efficient,and low-cost water oxidation catalysts has become especially important for the conversion of sunlight into green and accumulation chemical energy.One of the most promising water oxidation catalysts is polyoxometalates?POMs?because they can stand against fast,invertible,and stepwise multielectron-transfer reactions without changing their structures.They have been applied to broad fields,such as catalysis,material,and medicine.The application of POMs to catalysis is stimulated by their fascinating properties,including tunable acidity and redox properties,inherent resistance to oxidative decomposition,high thermal stability,and impressive sensitivity to light and electricity.These remarkable properties have a close relationship with their structures and compositions.To obtain special properties,their structure and constituent elements can be tuned artificially,in which the substitution of polyhedra,the variation of the heteroatom,and the arrangement patterns of the basic construction units are the most common pathways.Here,a series of Cu/Ni/Mn substituted POMs as the catalysts for water oxidation were further investigated under photocatalytic or electrochemical conditions.The main and new results are obtained and presented as following:1.Five different Cu-substituted POMs were synthesized and characterized.Their catalytic activities were evaluated with a Ru?bpy?32+/visible-light/Na2S2O8 photocatalytic water oxidation system.Among the tested POMs,only [Cu5?OH?4?H2O?2(A-?-SiW9O33)2]10-?Cu-1?exhibits water oxidation activity.Variables of the photocatalytic reaction conditions including catalyst concentrations,buffer pH values,dye concentrations and oxidant concentrations were systemically investigated to obtain the optimal conditions.Multiple experimental results confirm that Cu-1 is the dominant catalyst during water oxidation,such as UV/vis spectroscopy,FT-IR,XPS,DLS,laser flash photolysis,THpANO3 toluene extraction and capillary electrophoretic measurements.2.Nine different Ni-substituted POMs,as catalysts for light-driven water oxidation,were further researched at the same reaction conditions for comparison.Of the tested polyoxometalates,[{?-SiNi2W10O36?OH?2?H2O?}2]12-?Ni-1?gave the best water oxidation activity with a O2 yield of 27.2%.The optimum reaction conditions for water oxidation were obtained through four conditional experiments,varying the catalyst concentrations,buffer pH values,photosensitizer concentrations and oxidant concentrations Multiple experiments including laser flash photolysis,THpANO3 toluene extraction,capillary electrophoretic measurements and cyclic voltammetry confirm that Ni-1 is the dominant catalyst rather than Ni2+ ions?aq?or nickel oxide during water oxidation.We implemented electrochemical impedance spectroscopy and cyclic voltammetry to confirm the differences in the catalytic activity and catalytic behavior between Ni-1 and the equivalent Ni2+?aq?.3.A series of different Mn-containing POMs were synthesized and characterized by multiple experiments.The catalytic water oxidation abilities of these catalysts were investigated in photochemical and electrochemical manner.A definite catalytic activity order of water oxidation is obtained: [?Mn?H2O??3(SbW9O33)2]12-?Mn-1?> [Mn3?H2O?5(PW9O34)2]9-?Mn-2?> [Mn3?H2O?3(AsW9O33)2]12-?Mn-3?.The Mn3 POM based on Sb heteroatom?Mn-1?has the best water oxidation activity under photochemical and electrochemical systems.The heteroatoms in the polyoxoanion play important role for the water oxidation activity.Among the known water oxidation catalysts based on POM containing Mn,for the first time,we implemented electrochemical impedance spectroscopy method to confirm the differences in the electrocatalytic water oxidation properties between POM containing Mn and the equivalent Mn2+?aq?.Multiple experiments including UV-vis,IR,laser flash photolysis,THpANO3 toluene extraction,capillary electrophoretic measurements and electrochemical experiments confirm that Mn-1 is the dominant catalyst rather than Mn2+ ions or manganese oxide during the water oxidation process.In addition,to study the photocatalytic water oxidation ability of catalysts?Mn-1,Mn-2 and Mn-3?,the band gap structures of the three catalysts were investigated.The band gap structures of catalysts were verified by the measurements of electrochemistry and UV–vis diffuse reflectance spectra.4.A POM-based polynuclear copper cluster,[(?-SbW9O33)2Cu3?H2O?3]12-?Cu3POM?was synthesized and structurally characterized.This catalyst exhibits catalytic activity and stability towards water oxidation in neutral aqueous solutions.Multiple electrochemical experiments confirm that Cu3 POM maintain its structural integrity during the water oxidation process.The deposited copper oxide film was not found on the electrode surface of Cu3 POM,which was further verified by energy dispersive X-ray spectroscopy and scanning electron microscopy.Bulk water electrolysis catalyzed by CuCl2 was carried out under the same reaction conditions,in contrast with the bulk water electrolysis experiments over Cu3 POM,the electrode surface of CuCl2 was covered with depositing film.Meanwhile,we implemented multiple experiments to confirm the differences in the catalytic activity and catalytic behavior between Cu3 POM and the equivalent Cu2+?aq?.In addition,to explore this catalytic water oxidation process in detail,the kinetics studies for Cu3 POM were conducted by electrochemical methods. |
PDF Full Text Request