Preparation of Ti1Li3Al2-LDHs/g-C3N4 composites and its photocatalytic properties in CO2-toluene reaction system
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摘要: 光催化CO2还原是实现CO2绿色转化利用的重要途径之一，但一直受其反应转化效率低的制约。开发新的CO2还原反应体系和提高光催化剂的可见光利用率及光生电子与空穴的分离效率是解决上述问题的有效方法。本文利用甲苯作为底物，构建了光催化CO2-甲苯耦合反应的新体系，并通过静电组装法合成了Ti1Li3Al2-层状双氢氧化物(LDHs)/石墨相氮化碳(g-C3N4)复合光催化剂。重点研究了该复合光催化剂的光电性质及在CO2-甲苯耦合反应体系中的光催化反应特性。结果表明，在光催化CO2-甲苯耦合体系中，Ti1Li3Al2-LDHs/g-C3N4作用下，CO2被还原为CO，甲苯被氧化为苯甲醇、苯甲醛及苯甲酸苄酯，其中苯甲醛和苯甲醇的含量可达到4.80和4.70 mmol/gcat。这主要归因于Ti1Li3Al2-LDHs/g-C3N4中，g-C3N4将Ti1Li3Al2-LDHs在紫外区的吸收扩展到了可见光区，并提高了Ti1Li3Al2-LDHs的分散性，从而为光催化反应提供更多的活性位点；Ti1Li3Al2-LDHs/g-C3N4的界面处形成了S型异质结，有利于界面处的光生电子的转移，提高了其光生电子与空穴的分离效率，而甲苯可作为有机底物加快空穴的消耗速度促进了CO2还原反应的进行。为CO2与小分子有机物协同转化提供了一种新思路。
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关键词:
- Ti1Li3Al2-层状双氢氧化物(LDHs) /
- 石墨相氮化碳 /
- 光催化 /
- 异质结 /
- CO2-甲苯
Abstract: Photocataltyic reduction of CO2 is one of the promising routes in CO2 conversion and utilization, but the very low CO2 conversion rate is the biggest hurdle for the process. Developing a new CO2 reduction reaction system and improving the visible light utilization and separation efficiency of photogenerated electrons and holes are effective ways to solve the above problems. In this work, we designed a CO2-toluene coupling reaction system to promote the CO2 reduction reaction. The Ti1Li3Al2-layered dihydroxides (LDHs)/graphite phase carbon nitride (g-C3N4) composite with heterojunction structure were synthesized by electrostatic assembly method. And the photoelectric property and photocatalytic properties of Ti1Li3Al2-LDHs/g-C3N4 composite were explored in CO2-toluene reduction reaction system. The results show that CO2 is reduced to CO, and toluene is oxidized to benzyl alcohol, benzaldehyde and benzyl benzoate in the photocataltyic coupling reaction system. Benzaldehyde and benzyl alcohol content can reach 4.80 and 4.70 mmol/gcat. This is mainly because the g-C3N4 can extend the absorption of Ti1Li3Al2-LDHs from the ultraviolet region to the visible region, and improve the dispersion of Ti1Li3Al2-LDHs which provide more active sites for photocatalytic reactions. Moreover, the S-type heterojunction is formed in the interface of Ti1Li3Al2-LDHs/g-C3N4, which is conducive to the transfer of photogenerated electrons at the interface and improves the separation efficiency of photogenerated electrons and holes. And toluene can be used as an organic substrate to accelerate the rate of hole consumption and stimulate the CO2 reduction reaction. This work provides a new idea for the synergistic reaction between CO2 reduction and small molecular organics. -
图 2 Ti1Li3Al2-层状双氢氧化物(LDHs)的SEM图像
Figure 2. SEM images of Ti1Li3Al2- layered dihydroxides (LDHs)
图 3 Ti1Li3Al2-LDHs/石墨相氮化碳(g-C3N4)的生长示意图
Figure 3. Synthesis process of Ti1Li3Al2-LDHs/graphite phase carbon nitride (g-C3N4)
图 4 5∶1-Ti1Li3Al2-LDHs/g-C3N4 (a)、2∶1-Ti1Li3Al2-LDHs/g-C3N4 (b)、1∶5-Ti1Li3Al2-LDHs/g-C3N4 (c) 的TEM图像和Ti1Li3Al2-LDHs (d)、2∶1-Ti1Li3Al2-LDHs/g-C3N4 (e) 的TEM mapping图像
Figure 4. TEM images of 5∶1-Ti1Li3Al2-LDHs/g-C3N4 (a), 2∶1-Ti1Li3Al2-LDHs/g-C3N4 (b), 1∶5-Ti1Li3Al2-LDHs/g-C3N4 (c) and TEM mapping images of Ti1Li3Al2-LDHs (d), 2∶1-Ti1Li3Al2-LDHs/g-C3N4 (e)
Ratio before the composite name is the mass ratio of Ti1Li3Al2-LDHs to g-C3N4
图 5 不同样品的XRD图谱：Ti1Li3Al2-LDHs (a)、g-C3N4 (b) 和不同质量比的Ti1Li3Al2-LDHs/g-C3N4 (c)
Figure 5. XRD patterns of different samples: Ti1Li3Al2-LDHs (a), g-C3N4 (b) and different mass ratios of Ti1Li3Al2-LDHs/g-C3N4 (c)
图 6 g-C3N4、Ti1Li3Al2-LDHs和不同质量比的Ti1Li3Al2-LDHs/g-C3N4的FTIR图谱
Figure 6. FTIR spectra of g-C3N4, Ti1Li3Al2-LDHs and Ti1Li3Al2-LDHs/g-C3N4 with different mass ratios
图 7 g-C3N4、Ti1Li3Al2-LDHs和不同质量比的Ti1Li3Al2-LDHs/g-C3N4的N2吸附-解析等温线 ((a), (b)) 和孔结构分布图 (c)
Figure 7. N2 adsorption-desorption isotherms ((a), (b)) and the pore size distribution diagram (c) of g-C3N4, Ti1Li3Al2-LDHs and Ti1Li3Al2-LDHs/g-C3N4 with different mass ratios
STP—Standard temperature and pressure
图 8 g-C3N4、Ti1Li3Al2-LDHs和不同质量比的Ti1Li3Al2-LDHs/g-C3N4的UV-Vis DRS谱图 (a)、(αhν)2和hν的关系图 (b)、PL谱图 (c) 和莫特-肖特基(M-S)测试曲线 (d)
Figure 8. UV-Vis DRS spectra (a), relationship between (ahν)2 and hν (b), PL spectra (c) and Mott-Schottky (M-S) curves (d) of g-C3N4, Ti1Li3Al2-LDHs and Ti1Li3Al2-LDHs/g-C3N4 with different mass ratios
α—Absorption coefficient; hν—Photon energy; C—Capacitance
图 9 Ti1Li3Al2-LDHs/g-C3N4中S型异质结的电荷转移路径
Figure 9. S-scheme charge transfer mechanism of Ti1Li3Al2-LDHs/g-C3N4
EC—Conducation band; EV—Valence band; EF—Fermi energy
图 10 g-C3N4、Ti1Li3Al2-LDHs和不同质量比的Ti1Li3Al2-LDHs/g-C3N4的CO2-程序升温脱附(TPD)曲线 (a)、产物分布图 (b)和2∶1-Ti1Li3Al2-LDHs/g-C3N4不同时间的产物分布图 (c)、反应前后的XRD图谱 (d)
Figure 10. CO2-temperature programmed desorption (TPD) curves (a), product distribution (b) of g-C3N4, Ti1Li3Al2-LDHs and Ti1Li3Al2-LDHs/g-C3N4 with different mass ratios and product distribution at different time (c), XRD patterns before(used) and after(fresh) reaction (d) of 2∶1-Ti1Li3Al2-LDHs/g-C3N4
图 11 Ti1Li3Al2-LDHs/g-C3N4的光催化机制
Figure 11. Photocatalytic mechanism of Ti1Li3Al2-LDHs/g-C3N4
NHE—Normal hydrogen electrode
图 12 在Ti1Li3Al2-LDHs/g-C3N4上光催化CO2-甲苯耦合反应机制
Figure 12. Mechanism of photocatalytic CO2-toluene coupling reactions over Ti1Li3Al2-LDHs/g-C3N4
图 13 2∶1-Ti1Li3Al2-LDHs/g-C3N4中DMPO捕获的自由基的电子自旋谐振(ESR)光谱
Figure 13. Electron spin resonance (ESR) spectra of radicals captured by DMPO in 2∶1-Ti1Li3Al2-LDHs/g-C3N4
DMPO—5,5-dietyl-1-pyrroline-N-oxide
表 1 不同样品的比表面积及孔结构参数
Table 1. Specific surface area and pore structure parameters of different samples
Sample SBET/(m2·g−1) Vpore/(cm3·g−1) dpore/nm Ti1Li3Al2-LDHs 347.5 0.32 4.60 5∶1-Ti1Li3Al2-LDHs/g-C3N4 266.0 0.35 4.64 2∶1-Ti1Li3Al2-LDHs/g-C3N4 214.5 0.38 5.46 1∶5-Ti1Li3Al2-LDHs/g-C3N4 104.2 0.45 9.72 g-C3N4 85.6 0.54 12.06 Notes: SBET—BET surface area; Vpore—Pore volume; dpore—Pore size. 
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