Performance and mechanism of photocatalytic oxidation of low concentration coal-mine gas into methanol in mesoporous WO3 / H2O2 system

It is one of developing directions in coal-mine gas utilization to convert low concentration gas into liquid in- dustrial raw material methanol which can be easily stored and transported. How to realize the effective activation of methane and the selective generation of methanol under mild conditions are two key problems for methanol production by gas catalytic conversion. Mesoporous WO3 possessing high specific surface area was synthesized by using ordered porous silica KIT-6 as template and phosphotungstic acid-ethanol solution as precursor. The crystal composition,micro- structure,specific surface area,and pore structure characteristics of mesoporous WO3 were characterized by X-ray dif- fraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM) and low-temperature nitrogen adsorption-desorption technology. By using the synthesized WO3 as catalyst and H2 O2 as electron trapping a- gent and oxidant,the visible-light catalytic system of WO3 / H2 O2 was constructed. Visible-light catalytic activity for the partial oxidation of low concentration gas into methanol was systematically investigated in this WO3 / H2 O2 system. The results demonstrate that the initial concentration of methane,the irradiation intensity of visible light,and the added concentration of H2 O2 have effects on the methane conversion yield and methanol selectivity. There are different opti- mal H2 O2 concentrations for the simulated gas with different methane concentrations. For the imitative gas with a meth- ane concentration of 20% ,selective conversion of methane can be achieved over mesoporous WO3 in presence of opti- mal H2 O2 concentration of 13. 5 mmol / L. After 120 min of visible light irradiation,the conversion rate of methane rea- ches 24. 9% (8. 3 and 8. 9 times higher than single mesoporous WO3 or H2 O2 catalytic system) and the methanol se- lectivity is up to 82. 5% . The further increase of H2 O2 concentration can promote the conversion of methane,but the resultant excess hydroxyl radicals ( ·OH) lead to produce more by-products ethane ( C2 H6 ) and carbon dioxide (CO2 ),significantly reducing the methanol selectivity. The results of electron spin resonance show that the photoholes generated by visible-light exciting WO3 are the main species for methane activation. Methane molecules are firstly oxi- dized into methyl radicals (·CH3 ) via hydrogen extraction reaction by photoholes. H2 O2 is used as an electron trap- ping agent to enhance the generation of photoholes,improving the photo-activation efficiency of methane. In the mean- time,H2 O2 is reduced by photo-induced electrons on the conduction band of WO3 to produce hydroxyl radicals (·OH),which combine with ·CH3 to form product CH3 OH. That is,·OH is the primary oxidation species of meth- anol selective generation. These results will provide some novel ideas for clean utilization of low concentration coal- mine gas.