Ga-O双功能位点促进高选择性光催化甲烷直接转化制甲醇

Promotion of highly selective photocatalytic conversion of methane into methanol by Ga-O bifunctional sites

光催化甲烷(CH_(4))直接转化制甲醇(CH_(3)OH)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CH_(3)OH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Ga_(2)O_(3)光催化材料(α-Ga_(2)O_(3)和β-Ga_(2)O_(3)),并对其光催化CH_(4)直接转化制CH_(3)OH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-Ga_(2)O_(3)和β-Ga_(2)O_(3)表现出优异的光催化性能.相较于β-Ga_(2)O_(3),α-Ga_(2)O_(3)的光催化性能更优:反应2 h后,其CH_(4)转化率达4.5%,CH_(3)OH的生成速率与选择性分别高达372.8μmol/(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Ga_(2)O_(3)的Ga和O分别是活化CH_(4)和水分子(H_(2)O)的活性位点.得益于Ga-O双功能活性位点的协同作用,CH_(4)和H_(2)O活化产生的·CH_(3)和·OH可以直接结合,促进了CH_(3)OH的定向生成.由于α-Ga_(2)O_(3)比β-Ga_(2)O_(3)展现出更强的活化H_(2)O的能力,α-Ga_(2)O_(3)表现出更优异的光催化CH_(4)直接转化制CH_(3)OH性能.本文为设计高效的CH_(4)直接转化制CH_(3)OH催化材料打开了新的思路.

Methane(CH_(4)),the principal constituent of natural and shale gasses,possesses substantial global reserves.It is widely acknowledged that CH_(4),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 CH_(4) to yield high-value chemicals exhibits promising prospects for future development.Methanol(CH_(3)OH)is widely regarded as an optimal product for CH_(4) 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,CH_(4) is converted into syngas(CO and H_(2))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 CH_(4) to CH_(3)OH under mild conditions is essential.In recent years,photocatalytic technology has realized direct conversion of CH_(4) to CH_(3)OH under mild conditions.However,due to the tendency of the target product,CH_(3)OH,to undergo peroxide reactions,the selectivity is usually relatively low.Selective photocatalytic direct conversion of CH_(4) to CH_(3)OH is considered a“holy grail”reaction in the field of catalysis.In this work,we successfully synthesized Ga_(2)O_(3) photocatalytic materials with distinct crystal phases(α-Ga_(2)O_(3) and β-Ga_(2)O_(3))through solvothermal and calcination processes at high temperatures.Subsequently,their photocatalytic performance in the oxidation of CH_(4) to CH_(3)OH was evaluated.The experimental results highlight that both α-Ga_(2)O_(3) and β-Ga_(2)O_(3) exhibit exceptional photocatalytic performance in the oxidation of CH_(4) to CH_(3)OH under ambient conditions without requiring additional oxidants.Following a 2 h reaction,the selectivity towards methanol reached 82.7% and 80.3% for α-Ga_(2)O_(3) and β-Ga_(2)O_(3),respectively.The corresponding CH_(3)OH generation rates were 372.8 and 355.0μmol/(g h),while the CH_(4) conversion rates were 4.5%and 4.3%,and the carbon balance ratios were 89.8% and 92.1%,respectively.Compared with β-Ga_(2)O_(3),α-Ga_(2)O_(3) 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 Ga_(2)O_(3) represent active sites for CH_(4) and H_(2)O,respectively.CH_(4) undergoes activation on the O atoms of α-Ga_(2)O_(3) and β-Ga_(2)O_(3),forming CH_(3)O^(*)species due to its interaction with lattice oxygen atoms.Subsequently,the C-O bond of the intermediate CH_(3)O^(*)selectively cleaves,resulting in the formation of·CH_(3),which further combines with·OH to yield CH_(3)OH.The speculated pathway for CH_(4) conversion is as follows:CH_(4)+O(lattice)→CH_(3)O^(*)→·CH_(3)→CH_(3)OH.Conversely,the Ga atoms on α-Ga_(2)O_(3) and β-Ga_(2)O_(3) 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-Ga_(2)O_(3) and β-Ga_(2)O_(3) in the oxidation of CH_(4) to CH_(3)OH.Owing to its superior capability in activating water molecules into·OH,α-Ga_(2)O_(3) exhibits a higher methanol generation rate and selectivity than β-Ga_(2)O_(3).Compared with the reported literature,α-Ga_(2)O_(3) demonstrates the highest CH_(3)OH generation rate and exhibits high CH_(3)OH 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 CH_(4) to CH_(3)OH.