Fabrication of heterogeneous composite catalysts for carbon dioxide conversion

Abstract

With the excessive use of natural fossil fuels, an increasing amount of greenhouse gas carbon dioxide (CO2) has been produced and emitted, resulting the global warming. It is quite urgent to reduce the CO2 level for sustainable development. The CO2 photocatalytic conversion and electrochemical reduction methods have been regarded as promising strategies to convert CO2 and produce chemical feedstocks. However, it is still a challenge to fabricate efficient and durable catalysts for CO2 conversion. Polymeric carbon nitride (PCN) has attracted increasingly research attention in recent years due to its suitable band structure, facile preparation method, and chemical/physical stability. However, the application PCN is also restricted by its intrinsic drawbacks, including low specific surface area, fast recombination of photogenerated electron-hole pairs, and sluggish charge carrier migration. This thesis mainly focuses on the modification of PCN for improving CO2 conversion performance. To extend the light absorption range and suppress the recombination of photogenerated charge carriers of PCN. The single atom copper sites have been introduced into the framework of PCN. The CO2 photocatalytic performance of polymeric carbon nitride has been enhanced a lot after loading Cu sites, which exhibits 2.1 times higher CO production rate than that of pure carbon nitride. The Cu sites not only can enhance the visible light absorption, but also improve the ability of electrons transfer. The Cu atomic sites in the bulk phase of PCN could accelerate the photoinduced electrons transfer to the surface of photocatalysts inhibiting its fast recombination with holes. Furthermore, to enlarge the specific surface area of PCN, a facile NaCl-assisted method combined an in-situ single Cu atom loading strategy were proposed to successfully prepare the three-dimensional hollow polymeric carbon nitride (THPCN) modified with Cu atomic sites. The THPCN sample shows improved CO (1.6 µmol/g.h) and CH4 (0.11 µmol/g.h) conversion activity than that of bulk CN, which is due to the enlarged specific surface area. Furthermore, after loading Cu single atom sites, the obtained samples display larger CO yield rate of 2.8 µmol/g.h and CH4 of 0.27 µmol/g.h than THPCN. Apart from metal doping strategy, Ag and Pd bimetals have been loaded on PCN photocatalysts for CO2 conversion shown in chapter 4. The CO2 photocatalytic performance has been remarkably boosted up owing to excellent charge collection behavior and light absorption properties of PdAg bimetallic NPs. Among the obtained photocatalysts, the photocatalyst with 1:2 Pd-Ag molar ratio exhibited 5.42 μmol/g·h of CO production and 4.03 μmol/g·h of CH4. The production rate of CH4 is 40 times higher than that of bare CN photocatalyst. In addition to being used for photocatalysis, PCN was also adopted in CO2 electrochemical reduction reaction (CO2eRR) due to its strong CO2 affinity. In chapter 5, Ag nanoparticles decorated on sulfur-doped CN/CNT (Ag-S-CN/CNT) were prepared and displayed excellent activity and selectivity towards CO2eRR to CO. The Ag-S-CN/CNT showed a remarkable high current density of -21.3 mA cm-2 at -0.77 VRHE and the maximum FE (CO) of 91.4 % at -0.8 VRHE in H type cell.