FABRICATION OF HETEROGENEOUS COMPOSITE CATALYSTS FOR CARBON DIOXIDE CONVERSION

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 ?mol/g.h) and CH4 (0.11 ?mol/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 ?mol/g.h and CH4 of 0.27 ?mol/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 ? 0.01% at ?0.8 VRHE in H type cell.

随着化石燃料的过度使用，越来越多的温室气体二氧化碳（CO2）产生和被排放，加速了全球变暖。控制和降低CO2浓度已成为人类可持续发展的当务之急。CO2的光催化转化和电化学还原被认为是转化CO2和生产化学原料的有前途的方法。然而，制造高效耐用的催化剂仍然是一个巨大挑战。 氮化碳（PCN）材料，由于其合适的能带结构、简便的制备方法，化学/物理性质稳定等优点近年来受到越来越多的研究关注。然而，PCN的应用也受到其固有缺点的限制，包括低比表面积、光生电子-空穴对的快速复合以及电荷载流子迁移缓慢。本论文主要研究PCN的改性以提高其对CO2转化性能的影响。 通过将Cu单原子位点引入到 PCN 框架中，氮化碳的CO2光催化性能得到了很大提高，其CO产率比纯氮化碳高2.1倍。Cu位点的引入不仅可以增强可见光吸收，还可以提高电子转移能力。PCN体相中的Cu原子位点可以加速光致电子转移到光催化剂表面，抑制其与空穴的快速复合。此外，为了扩大PCN的比表面积，NaCl模板结合Cu单原子负载策略，成功制备了负载Cu位点的三维中空氮化碳（THPCN）。由于比表面积增大，THPCN 样品的CO (1.6 ?mol/g.h) 和 CH4 (0.11 ?mol/g.h) 转化效率比纯CN有所提高。此外，在负载 Cu 单原子位点后，获得的样品比THPCN有着更高的CO (2.8 ?mol/g.h) 和CH4 (0.27 ?mol/g.h) 产率。 除了金属掺杂策略外，Ag和 Pd双金属颗粒被负载在 PCN 表面用于CO2 的光催化转化，如第4章所示。由于PdAg双金属纳米颗粒优异的电荷收集行为和光吸收性能，负载双金属颗粒的氮化碳的CO2光催化性能得到显着提升。在制备的催化剂中，Pd-Ag摩尔比为1:2时，样品的性能最为优异，CO产量为5.42 μmol/g·h，CH4生成量为4.03 μmol/g·h，CH4的产率比CN提高了近40倍。 除了应用于光催化，PCN 还因其强大的 CO2亲和力而被用于CO2电化学还原反应 (CO2eRR)。在第5章中，Ag纳米颗粒成功负载在硫掺杂的CN/CNT上（Ag-S-CN/CNT），并在CO2eRR中表现出对CO的优异活性和高选择性。在H 型反应池中，Ag-S-CN/CNT在-0.77 VRHE 时电流密度为-21.3 mA cm-2，在-0.8 VRHE 时，CO法拉第效率为91.4 ± 0.01%。