Atomic Level Insights for Photocatalytic CO2 Reduction

Abstract

As the concentration of greenhouse gases in the atmosphere continues to rise, so does average global temperature. This process is irreversible over a short time. The development of renewable clean energy is important therefore to survival and sustainability. As a potential practical method of solar energy utilization, photocatalysis can convert CO2 into high-value-added fossil fuels and assist to obviate global temperature and solve energy shortage. Photocatalytic CO2 reduction (PCRR) is an artificial photosynthesis, that includes three steps: 1) Light absorption, 2) Charge separation and transfer, and 3) Surface redox reaction. However, high C=O double bond energy and the symmetrical molecular structure of CO2 molecules make it necessary to overcome a significantly high energy barrier for adsorption and activation. Therefore, a simple, environmentally friendly and universal method is urgent to be developed to activate the catalyst surface and significantly boost affinity for CO2 molecules. Additionally, photocatalytic CO2 reduction reaction involves complex, multi-electron transfer paths, leading to complex catalytic products and reaction kinetics. However, the construction of photocatalytic reaction systems that exhibit excellent performance, stability and low cost and are environmentally benign is a significant practical challenge. This Thesis aim is to determine the fundamentals of the atomic level and to design novel nanostructured photocatalysts for CO2 reduction reactions. Importantly, this involves detailed investigations on structure and performance, especially for charge transfer and surface reactions that can be reliably used to guide design of catalysts. The significance, context and scope of this Thesis is presented in Chapter 1. A critical review of research progress for CO2 photoreduction from atomic level understanding is provided in Chapter 2. This chapter critically evaluates the challenges and discusses atomic level active sites for PCRR, including defects, single atoms, functional groups and frustrated Lewis pairs and relationships between reactive sites and PCRR performance and impacts on selectivity, stability, efficiency and reactivity. Chapter 3 focuses on synthesizing reactive sites on the surface of transition metal dichalcogenides (TMDs) to alter the inert surface. A heterojunction between CdS nanoparticles and ultra-thin ReS2 nanosheets (6 nm thickness) is synthesized and the mechanism for CO2 photoreduction is determined for this catalytic system. Photoelectrons separation and transfer and in situ formed defects on the surface of ReS2 are confirmed to act as reactive sites to boost photocatalytic performance. Carbon contaminations have a significant influence on photocatalytic performance because carbon-contained products generated via CO2 conversion remain at a low level. In Chapter 4, a targeted series of experiments are described to identify carbon impurities and to establish the influence of carbon impurities on subsequent performance tests. Based on findings from these, reliable experimental protocols to obviate potential carbon contamination effect are proposed in detail. In Chapter 5, a straightforward strategy is developed for synthesis of surface defective bismuth oxide nanosheets. The (010) surface of nanosheets are actively regulated via oxygen vacancies. The surface-regulated Bi2MoO6 nanosheets exhibit practically promising PCRR performance. Findings from judiciously combined theoretical computations, kinetics analyses and in situ infrared spectroscopy are used to confirm that surface unsaturated metal atoms serve as the active sites, and that the hydrogenation of *OCH3 for CH4 formation is the rate-limiting step. Findings are shown to increase fundamemtal understanding of the regulated surface, and to form a basis for large-scale application. In the concluding experimental chapter, Chapter 6, dual single atoms for efficient CO2 conversion to value-added chemicals are explored. In situ characterizations and simulations evidence complex surface reaction and free energy diagram. Charge transfer is determined via in situ X-ray Photon Spectra (XPS) to demonstrate that the photogenerated electrons transfer direction is reversed in dual single atoms system compared with single-atom system. Findings are shown to develop understanding of the charge transfer between dual single atoms and single-atom research and mechanism. The conclusions available from findings from this research on CO2 photoreduction, togther with a perspective are presented in Chapter 7.