Study On Photocatalytic Methane Conversion Reaction And Mechanism By Tungsten Trioxide

Under mild conditions,the conversion of methane（CH₄）into liquid oxygenates is of great significance to the sustainable development of energy and environment.Photocatalytic CH₄conversion technology utilizes the carrier separation driven by light energy to excite the surface species to generate active oxygen,and then activate the C-H bond of CH₄ molecules,thus realizing the direct conversion of CH₄ under mild conditions.However,this process still faces the bottleneck of low activity and poor selectivity.Tungsten trioxide（WO₃）has a suitable band gap,excellent photocatalytic performance and strong valence band hole oxidation ability,but its conduction band position is slightly positive,photo-generated charges are easy to recombine,and its photocatalytic CH₄ conversion ability is limited.Therefore,this paper introduced defect modification and supported metal cocatalyst to regulate the photo-generated charge separation kinetics of WO₃,and improved the photocatalytic CH₄ conversion reaction performance.The main contents are as follows:Firstly,WO₃ modified with palladium nanoparticles and oxygen vacancies as double active sites drives the conversion of methane to O₂ at room temperature.Compared with WO₃,the optimized Pd_0.5-def-WO₃ photocatalyst increased the yield of oxygenates by nearly 33 times,with the yield of 7018μmol g^-1 h^-1,and the high selectivity to primary products（CH₃OH and CH₃OOH）was 81%,which was superior to most of the previously reported catalysts.According to the shift of Pd_3d to high binding energy,in-situ XPS spectrum proves that Pd nanoparticles are hole acceptors.In-situ EPR spectra show the enhancement of OVs signal,which proves the role of OVs as an electron acceptor.Therefore,the double modification of Pd nanoparticles and OVs realizes efficient charge separation,which contributes to the superior activity and selectivity of methane conversion.Secondly,using the defect-rich WO₃ synthesized by hydrothermal method as precursor,the content of defects in WO₃ was optimized by adjusting the annealing time of catalyst in air atmosphere.On this basis,WO₃-based photocatalyst co-modified by copper species and oxygen defects was successfully prepared.Under room temperature and light conditions,highly efficient directional conversion of CH₄ to formaldehyde（HCHO）was realized,with a selectivity of 100%and a HCHO yield of 1120μmol g^-1 h^-1.The reaction mechanism shows that the appropriate defect concentration narrows the band gap of the catalyst and expands the light absorption range.At the same time,the synergistic effect of Cu and oxygen vacancies promotes the rapid migration of photogenerated electrons and inhibits the recombination of photogenerated carriers.In the above research,we successfully prepare the corresponding catalysts,and explore the photocatalytic CH₄ conversion performance and mechanism of the two modified catalysts by XRD,TEM,EPR,XPS,GC-MS and other characterization methods.The first research work proves that Pd as hole acceptor and oxygen vacancies as electron acceptor synergistically promote the catalytic activation of CH₄.The second research work shows that an appropriate amount of oxygen vacancies could promote the catalytic conversion of CH₄ and achieve high selectivity of products.The two modification strategies achieve the purpose of high-efficiency activation and directional activation of CH₄,respectively.