Construction Of Carbon Dots Modified Co-based Catalysts And Exploration Of Their Catalytic Performance

With the rapid development of technology,more and more new inventions and technologies have brought great convenience to current society.But most technologies rely on carbon-based fuels such as oil,coal and natural gas.The large consumption of these resources not only caused a shortage of energy,but also caused a bad impact on the ecological environment.As a result,searching for renewable energy sources and exploring new energy conversion systems have become more and more important in current society.Water decomposition is one of the energy conversion systems that has been studied a lot in recent years.However,the high overpotential of electrocatalytic water oxidation and the low efficiency of photocatalytic water decomposition hinder the further application of water decomposition.Therefore,it is urgent to find low-cost,stable and efficient catalysts to solve the above problems.The transition metal cobalt has become a research focus in the fields of electrocatalysis and photocatalysis due to its high reserves,low cost and high activity.However,some cobalt-based catalysts often face problems such as poor conductivity,fast recombination rate of photo-generated electrons and holes,and easy deactivation when used in photo/electrocatalysis.Doping or recombination is an important strategy to improve the catalytic performance of cobalt-based catalysts.In this paper,cobalt-based catalysts are used as the main research object and they are further modified with carbon dots to improve the catalytic activity of the composite catalysts.Three carbon dots modified cobalt-based composites are prepared,including cobalt phosphide/carbon dots electrocatalyst(CoP/CDs),cobalt phosphate-carbon dots photocatalyst(CoPi-CDs)and carbon dots modified cobalt oxide-titanium silicon molecular sieve photocatalyst(CoO-TS-1-CDs).The transmission electron microscopy,X-ray photoelectron spectroscopy and other instruments were used to characterize them.The electrocatalytic water oxidation performance or photocatalytic water splitting of the composites were also tested and their catalytic mechanisms were further studied.The research contents are as follows:(1)CoP/CDs composites were fabricated through hydrothermal reaction,high-temperature phosphating and stirring methods,and the electrocatalytic water oxidation properties of the samples in alkaline solutions were studied.In addition,by changing the content of carbon dots in the composite,the catalytic performance of the composite is regulated.The experimental results show that when the content of carbon dots in the composite is 6 mg(28.79 wt.%C),the obtained CoP/CDs composite exhibits the optimized electrocatalytic activity and stability(in 1 M KOH,the overpotential is 400 mV at the current density of 10 mA·cm-2).The synergistic effect between cobalt phosphide and carbon dots is the main reason for the improved electrocatalytic performance of cobalt phosphide.(2)The fast carrier recombination rate of cobalt phosphate prevents its application in photocatalytic water splitting.In order to suppress the photo-generated electron-hole recombination rate of cobalt phosphate,cobalt phosphate-carbon dot composites(CoPi-CDs)with different content of CDs were designed and fabricated through the precipitation method,and their application in photocatalytic water spitting was explored under visible light without other cocatalysts and sacrificial agents.It has been confirmed that when the concentration of carbon dots in the composite is 0.002 gCDs/gcatalyst,the optimal composite exhibits a hydrogen production rate of approximately 0.592 ?mol h-1 and an oxygen generation rate of 0.258 ?mol h-1,about 33 times that of pure CoPi.In this system,the photo-generated electrons of CoPi are transferred and redistributed on the surface of CDs,thereby inhibiting the recombination of photo-generated electrons and holes,and effectively promoting the performance of photocatalytic water splitting.(3)In order to solve the problem that cobalt oxide(CoO)is easily agglomerated and deactivated during the photolysis of water,titanium-silicon molecular sieve(TS-1)was used as a carrier,and CoO nanoparticles were supported on TS-1 through facile adsorption and calcination steps.The photocatalytic activity of the synthesized composites were studied without other cocatalysts and sacrificial agents.Moreover,combined with the the strong adsorption property of TS-1 to H2O2,the utilization of the product of water splitting(H2O2)was further explored.The results manifest that the fabricated CoO-TS-1 has a good photocatalytic activity and stability.The maximum H2 generation rate is 1460 ?mol h-1 gCoO-1,and the H2O2 generation rate is 1390 ?mol h-1 gCoO-1.After the photocatalytic reaction,CoO-TS-1 adsorbed with H2O2 exhibited good catalytic activity and stability in the cyclohexane oxidation reaction.In this system,CoO is employed as the active site for photocatalytic water splitting,and TS-1 is used as a H2O2 adsorbent and the carrier to inhibit CoO agglomeration.In order to solve the thermal instability of CoO in the CoO-TS-1 system,carbon dots with thermal conductivity were further used to modify the CoO-TS-1 composite to improve the performance of the catalyst.