介孔WO3H-2O2 光催化氧化低浓度瓦斯制甲醇性能与机理

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

  • 摘要: 将低浓度瓦斯转化为易于储运的液态工业原料甲醇是其综合利用的一个发展方向,如何实现温和条件下甲烷的有效活化与甲醇的选择性生成是瓦斯催化转化制甲醇的两个关键问题。本文以有序多孔二氧化硅KIT-6为模板剂,磷钨酸-乙醇溶液为前驱物,合成了具有高比表面积的介孔三氧化钨(WO3),通过X-射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)及低温氮吸附-脱附技术对介孔WO3的晶型组成、微观形貌、比表面积及孔结构特性进行表征,以所制介孔WO3为催化剂、双氧水(H2O2)为电子捕获剂与氧化剂,构建了WO3/H2O2可见光催化体系,并对该体系氧化低浓度瓦斯合成甲醇的性能进行了系统研究。结果表明:甲烷初始体积分数、可见光照射强度、H2O2添加等均会影响甲烷转化率与甲醇选择性,且不同甲烷体积分数模拟瓦斯有不同的最佳H2O2;对于甲烷体积分数为20%的模拟瓦斯,介孔WO3在优选的H2O2溶液(浓度为13.5 mmol/L)中可实现甲烷的选择性转化,可见光照射120 min甲烷转化率为24.9%(是仅使用介孔WO3或H2O2体系的8.3与8.9倍),且甲醇选择性高达82.5%,继续增大H2O2可进一步促进甲烷的转化,但由此产生的过量羟基自由基(·OH)导致生成更多的副产物乙烷(C2H6)和二氧化碳(CO2),使甲醇选择性显著降低;电子自旋共振测试结果表明WO3吸收可见光产生的光空穴是活化甲烷分子的主要物种,甲烷分子首先经由光空穴抽氢反应生成甲基自由基(·CH3),H2O2一方面作为电子捕获剂增强光空穴的生成,提高甲烷光活化效率,同时被WO3导带电子还原生成羟基自由基(·OH),并进一步与·CH3相结合生成产物甲醇,即·OH是甲醇选择性生成的主要氧化物种。上述结果为煤矿低体积分数瓦斯的清洁利用提供新思路。

     

    Abstract: 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.

     

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