摘要
光催化甲烷(CH4)直接转化制甲醇(CH3OH)被誉为催化界的"圣杯反应".然而,由于反应过程中目标产物容易发生过氧化反应,实现CH3OH的定向转化仍然面临巨大挑战.本文通过溶剂热法和高温煅烧法成功地制备了不同晶相的Ga2O3光催化材料(α-Ga2O3和β-Ga2O3),并对其光催化CH4直接转化制CH3OH的性能进行评价.测试结果显示在室温常压,且不额外添加其他氧化剂的情况下,α-Ga2O3和β-Ga2O3表现出优异的光催化性能.相较于β-Ga2O3,α-Ga2O3的光催化性能更优:反应2h后,其CH4转化率达4.5％,CH3OH的生成速率与选择性分别高达372.8 μmol/(g h)和82.7％.通过原位红外光谱(in situ DRIFTS)和原位电子自旋共振谱(in situ EPR)对反应机理进行了分析,发现Ga2O3的Ga和O分别是活化CH4和水分子(H2O)的活性位点.得益于Ga-O双功能活性位点的协同作用,CH4和H2O活化产生的·CH3和.OH可以直接结合,促进了CH3OH的定向生成.由于α-Ga2O3比β-Ga2O3展现出更强的活化H2O的能力,α-Ga2O3表现出更优异的光催化CH4直接转化制CH3OH性能.本文为设计高效的CH4直接转化制CH3OH催化材料打开了新的思路.
Abstract
Methane(CH4),the principal constituent of natural and shale gasses,possesses substantial global reserves.It is widely acknowledged that CH4,a prevalent fossil fuel,releases significant quantities of carbon dioxide upon combustion.Notably,methane assumes significance not only as a vital fossil fuel but also as a crucial precursor for synthesizing valuable chemicals.The oxidation of CH4 to yield high-value chemicals exhibits promising prospects for future development.Methanol(CH3OH)is widely regarded as an optimal product for CH4 conversion in high-value chemicals,owing to its advantageous properties as a liquid chemical,facilitating convenient storage and transportation.Currently employed in industrial settings,methanol synthesis from methane involves an indirect approach.Initially,CH4 is converted into syngas(CO and H2)under high temperature and pressure conditions.Subsequently,this syngas undergoes Fischer-Tropsch synthesis to yield methanol.The present method is characterized by a complex process flow,high energy consumption,and elevated reaction costs,which pose significant barriers to sustainable development and implementation.Therefore,developing low-energy consumption and low-cost methods to achieve selective oxidation of CH4 to CH3OH under mild conditions is essential.In recent years,photocatalytic technology has realized direct conversion of CH4 to CH3OH under mild conditions.However,due to the tendency of the target product,CH3OH,to undergo peroxide reactions,the selectivity is usually relatively low.Selective photocatalytic direct conversion of CH4 to CH3OH is considered a"holy grail"reaction in the field of catalysis.In this work,we successfully synthesized Ga2O3 photocatalytic materials with distinct crystal phases(α-Ga2O3 and β-Ga2O3)through solvothermal and calcination processes at high temperatures.Subsequently,their photocatalytic performance in the oxidation of CH4 to CH3OH was evaluated.The experimental results highlight that bothα-Ga2O3 and β-Ga2O3 exhibit exceptional photocatalytic performance in the oxidation of CH4 to CH3OH under ambient conditions without requiring additional oxidants.Following a 2 h reaction,the selectivity towards methanol reached 82.7％and 80.3％for α-Ga2O3 and β-Ga2O3,respectively.The corresponding CH3OH generation rates were 372.8 and 355.0 μmol/(gh),while the CH4 conversion rates were 4.5％and 4.3％,and the carbon balance ratios were 89.8％and 92.1％,respectively.Compared with β-Ga2O3,α-Ga2O3 exhibits superior photocatalytic performance.Then,the reaction mechanism was investigated using in situ diffuse reflection Fourier transform infrared spectroscopy(in situ DRIFTS)and in-situ electron paramagnetic resonance spectroscopy(in situ EPR).This analysis demonstrates that the O and Ga atoms on Ga2O3 represent active sites for CH4 and H2O,respectively.CH4 undergoes activation on the O atoms of α-Ga2O3 and β-Ga2O3,forming CH3O*species due to its interaction with lattice oxygen atoms.Subsequently,the C-O bond of the intermediate CH3O*selectively cleaves,resulting in the formation of CH3,which further combines with OH to yield CH3OH.The speculated pathway for CH4 conversion is as follows:CH4+O(lattice)→ CH3O*→ CH3 → CH3OH.Conversely,the Ga atoms on α-Ga2O3 and β-Ga2O3 serve as the location where water molecules undergo activation to form OH radicals.The Ga-O bifunctional active site synergistically enhances the photocatalytic performance of both α-Ga2O3 and β-Ga2O3 in the oxidation of CH4 to CH3OH.Owing to its superior capability in activating water molecules into OH,α-Ga2O3 exhibits a higher methanol generation rate and selectivity than β-Ga2O3.Compared with the reported literature,α-Ga2O3 demonstrates the highest CH3OH generation rate and exhibits high CH3OH selectivity under room temperature and normal pressure conditions without the addition of extra oxidants,even surpassing the catalytic performance of some metal-modified materials.This study presents novel insights into the design of efficient catalytic materials for the oxidation of CH4 to CH3OH.
基金项目
国家自然科学基金(22209135)
国家自然科学基金(22209136)
中央引导地方科技发展资金(22ZYZYTS0231)
四川省自然科学基金(2022NSFSC1264)
中国博士后科学基金(2022M722635)
四川省博士后创新人才支持项目()
西南石油大学研究生科研创新基金(2020CXZD16)
NETL
NSTL
维普
万方数据