The Study On The Photo-oxidation Performance For The Conversion Of Low Concentration Coalbed Methane To Methanol Over Mesoporous Tungsten Oxide-based Catalysts

In order to reduce the occurrence of gas disasters during the process of coal mining,gas extraction is required in coal mines.However,most of the low concentration gas extracted from coal mines is discharged directly into the atmosphere due to the low concentration of methane and the difficulty in utilization,which not only causes energy waste but also pollutes the atmospheric environment.The direct conversion of low-concentration gas into high-value-added liquid methanol for storage and transportation is considered to be the best strategy for its efficient utilization.How to achieve the activation of methane and selective generation of methanol under mild conditions is the key to the conversion of low concentration gas to methanol.In this paper,ordered mesoporous tungsten oxide（WO₃）was prepared by an inverse replication method with mesoporous silica KIT-6 as template and phosphotungstic acid as tungsten source.The crystal composition,microstructure,specific surface area and pore structure of mesoporous WO₃ were characterized by X-ray diffraction,scanning electron microscopy,transmission electron microscopy and low-temperature nitrogen adsorption.Using mesoporous WO₃ as visible light catalyst and H₂O₂ as electron trapping agent,the photocatalytic conversion system of low-concentration gas was constructed,and the conversion performance of direct oxidation of low-concentration gas to methanol was systematically studied.The results demonstrate that the initial concentration of methane,H₂O₂ concentration and visible-light intensity all affected the methane conversion and methanol selectivity,and the simulated gas conversion system with different methane concentrations has the corresponding optimal H₂O₂ concentration.For the simulated gas with an initial methane concentration of 20%,the methane conversion and methanol selectivity are the highest when the H₂O₂ concentration is 16 mM.After 2h of visible light irradiation,the methane conversion rate reached 21.9% and methanol selectivity reached 78.5%.Further increase of H₂O₂ concentration can promote the methane conversion,but the target product methanol would be over-oxidized by hydroxyl radicals（·OH）into carbon dioxide.Electron spin resonance（ESR）technique was used to explore the reaction mechanism.The photoholes generated by visible-light excitation of WO₃ can activate the methane molecules into methyl radicals（·CH₃）,and the H₂O₂ not only can be used as electron capture agent to improve the photo-activation efficiency of methane,but also can be reduced by photoinduced electrons on the conduction band of WO₃ to produce hydroxyl radicals（·OH）,then ·CH₃ and ·OH react to form methanol via radical reaction.In order to further improve the photocatalytic performance of mesoporous WO₃,iron（Fe）,nickel（Ni）,cobalt（Co）and copper（Cu）oxide species as cocatalysts were anchored on the surface of mesoporous WO₃ via impregnation method.Based on the experimental results of WO₃/H₂O₂ system,the optimization of the cocatalysts was completed.With the H₂O₂ concentration of 16 m M,the methane conversion rate over1.20% FeO_x/WO₃ reached 33.8%,and methanol selectivity was up to 90.0%,the methane conversion performance of 1.20% FeO_x/WO₃ is significantly higher than that of mesoporous WO₃.The results of XRD,TEM,and elemental mapping show that the introduced iron species with amorphous structure are highly dispersed on the surface of mesoporous WO₃.The XPS results indicate that the introduced iron species are mainly iron oxides（FeO_x）.The performance enhancement mechanism of FeO_x/WO₃ catalysts was analyzed by using electron spin resonance,radical quenching test,transient photocurrent response,and photoluminescence spectrum.The improvement of methane conversion and methanol selectivity is mainly attributed to the fact that the introduction of FeO_x can significantly accelerate the separation of photogenerated electrons and holes,producing more active species to activate methane.Meanwhile,H₂O₂ was reduced to ·OH by the photoelectrons transferred to the surface of FeO_x,and then combined with ·CH₃ generated by CH₄ activation to produce methanol.The study on photocatalytic oxidation of low-concentration gas to methanol not only provides novel ideas for the clean utilization of gas and the safety production of coal mine,and also provides theoretical support for comprehensive development and utilization of methane-rich resources,such as coalbed methane,shale gas and combustible ice.