The Preparation And Catalytic Properties Of Novel Platinum And Strontium Titanate Nanomaterials

Alone with the rapid development of global industrialization,the demand for energy by people is increasing daily.Fossil fuels is one of the ways for energy.However,the large amount of fossil fuels leads to excessive emissions of greenhouse gases such as CO₂,which can cause a series of problems such as greenhouse effect,iceberg melting,sea level rise,etc.The development of new energy technologies that can replace fossil fuels has always been a hot research topic for researchers.Solar energy has inexhaustible advantages.It is one of the methods for developing new energy technology to convert cheap and easily available small molecular compounds?such as CO₂ and N₂?into economically valuable chemicals?such as alcohols and NH₃?by electrocatalysis and photocatalysis to realize efficient energy conversion.Nanomaterials?such as platinum nanoparticles and strontium titanate?have excellent photoelectrochemical properties,which can be used to realize the above conversion.However,the application of platinum?Pt?or strontium titanate?STO?in CO₂ reduction/nitrogen fixation has some limitations.For example,Pt has strong adsorption capacity for CO and also has high activity for hydrogen evolution.STO has a narrow response range to the spectrum and a high recombination rate of photogenerated electron-hole pairs.Therefore,it is necessary to modify Pt and STO to achieve efficient CO₂ reduction or nitrogen fixation.Many studies have shown that the catalytic properties of Pt can be significantly improved through nanoengineering its surface structure and morphology.In addition,in the modification strategy of STO,some researchers found that oxygen vacancies?OVs?and carbon quantum dots?CQDs?can effectively enhance the response range of materials to the spectrum and reduce the recombination efficiency of photogenerated electron-hole pairs.Based on the advantages of the above modification strategies,some modifications have been carried out on Pt or STO in this thesis to realize efficient CO₂ reduction/nitrogen fixation on Pt or STO.The research content is divided into two parts.One is electrochemical CO₂ reduction after Pt nanoengineered.The other is based on nano-mesoporous strontium titanate material photocatalytic nitrogen fixation.The main contents are as follows:1.A two-step method is used to prepare hierarchically-ordered platinum nanochannel array?HPNAs?.This fabrication method is not inherently area limited and can be easily scaled up.The HPNA with three dimensional electrocatalytic interfaces which have macropore channels in combination with mesoporous walls.The macrochannel arrays enable efficient mass transfer to and from the interfaces,while the mesoporous walls provide a large number of highly catalytic active sites for CO₂ adsorption and reduction.The HPNA exhibits unique three dimensional electrocatalytic interfaces with high activity and selectivity in CO₂conversion producing methanol and ethanol as the dominant liquid products.The Faradaic efficiency and yield for alcohol respectively.Electrochemical characterization?Linear sweep voltammetry,Chronopotential,Anodic stripping voltammetry,Cyclic voltammetry?of HPNA are tested to investigate the reasons for its excellent electrocatalytic performance.2.Carbon quantum dots?CQDs?with uniform morphology,particle size of3.5?6 nm,and surface rich in hydrophilic functional groups of-OH and-COOH are prepared by electrochemical method.Through the analysis of CQDs by ultraviolet spectrophotometer?UV?,it is found that the absorption range of CQDs can be extended to the visible region.By comparing the luminescence emission spectra of the CQDs without and with the quenchers,it is proved that CQDs is not only an excellent electron donor but also an excellent electron acceptor in the visible region.Nano-mesoporous strontium titanate?STO?is successfully synthesized by hydrothermal/solvothermal method with high purity and no other impurity components.A number of single factor experiments are carried out to systematically study the effects of material morphology,light intensity,photocatalyst concentration,hydrogen heat treatment temperature,carbon quantum dot content in heterostructure,sacrificial medium and other factors on photocatalytic nitrogen fixation performance.The following conclusions are drawn:?1?When the light intensity is 2 suns(200 m W·cm^-2)and the photocatalyst concentration is 0.25 g·L^-1,the unit ammonia yield of photocatalyst is the highest.?2?The photocatalytic nitrogen fixation performance of nano-mesoporous STO with rough surface is better than that of STO with smooth surface.?3?The photocatalytic nitrogen fixation performance of nano-mesoporous STO with rough surface increases with the increase in hydrogen heat treatment temperature.When the hydrogenation temperature?1000?,the photocatalytic nitrogen fixation performance basically remains unchanged.?4?The nitrogen fixation performance of carbon quantum dots/strontium titanate?CQDs/STO?heterostructures increases at first and then basically remains unchanged with the CQDs loading.In order to maximize the utilization rate of CQDs,the CQDs loading is 3 wt%.Based on the results of single factor experiments,the synthesis scheme of the materials is optimized,and the mesoporous strontium titanate rich in oxygen vacancies?OVs?and the heterostructures of strontium titanate rich in oxygen vacancies and carbon quantum dots are prepared.The synthesized materials are characterized by SEM,TEM,XRD,Raman,XPS,IR,PL,UV,etc;and the reasons for the excellent nitrogen fixation performance of the materials are explained.The photocatalytic nitrogen fixation performance of the materials is tested in ¹⁴N₂,¹⁵N₂ and Ar atmospheres at normal temperature and pressure respectively,and the products are analyzed by UV and NMR.The experimental results show that compared with pure mesoporous STO,the nitrogen fixation performance of mesoporous STO photocatalyst rich in oxygen vacancies is nearly doubled.The CQDs/STO heterostructure obtained by combining pure mesoporous STO,OVs and CQDs has a nitrogen fixation performance of nearly6 times higher than that of pure mesoporous STO.CQDs/STO heterostructure photocatalyst has excellent photocatalytic nitrogen fixation performance.