Research On Methane Conversion By Tungsten Oxide Heterostructure Via Photocatalysis

Photocatalysis is regarded as a new and clean technique which makes use of solar energy to transform photo energy to chemical energy.Methane is one of the warm house gases resulting in severe global warming effect,in general,the preparation of valued chemical products is via high temperature and high pressure in harsh condition.Due to the easier overoxidation of methane oxygenates,the whole process is high cost and low selectivity.If the transformation from methane to high-valued liquid products could be achieved via milder condition,it would be commercial value and beneficial for reducing environment pollution.Photocatalysts is the important part of a reaction,among the wide variety of semiconductors,tungsten trioxide is favored for its stability,nontoxic and environment friendly characteristics.However,there is carrier recombination in single catalysts and the common measurement is to construct heterojunction to improve the photocatalytic efficiency.The dissertation focused on the heterojunction construction by loading noble metals,detected relevant products and characterized catalysts using necessary instruments,then proposed reaction mechanism.1.Preparing Au loading photocatalysts by hard-template and in-situ chemical reduction methods,then reacted at room temperature and achieved the direct transformation from methane to formaldehyde.The optimized catalysts could reach 7202μmol g^-1 formaldehyde production with selectivity of 100%.Besides,using Mott-Schottky test and UV-vis diffusion reflectance spectroscopy test to analyze the band structure of photocatalysts,then investigating the function of metals by fluorescence test and electrochemical impedance spectroscopy test,and the isotope label test was used to infer the product formation and mass spectroscopy was used to identify the products.Finally,methane was activated by the crystal-O and converted to formaldehyde directly,and oxygen acted as oxidant and supplied for the crystal oxygen,besides,gold acted as conductor to promote the separation of electrons and holes.2.Synthesizing Pt loading nanosized tungsten trioxide by in-situ chemical reduction,and the reaction performed in room temperature irradiated by Xe lamp with oxygen to produce liquid products which accounted for 10.135 mmol g^-1 with selectivity of 98.3%.When changing Xe lamp to single light,the high quantum efficiency of 10.79%could be achieved at 365 nm.The band structure of photocatalysts could be measured by UV-vis absorption and diffusion reflectance spectroscopy and Mott-Schottky test,and the detection of free radicals could be performed by electron spin resonance.The catalysts showed broaden light absorption area and the loading of Pt nanoparticles could transfer electrons immediately and act as new active sites to make use of electrons.In conclusion,the decoration of Pt nanoparticles could act as new active sites to realize high light utilization and large productions.