title: Design of Photocatalysts for Efficient Heterogeneous and Heterogeneous/Homogeneous Photocatalytic Carbon Dioxide Reduction Systems
Creators: name: Jiang, Shuaiyu
Publisher: name: Griffith University
Year: 2021
Subjects: subject: heterogenous photocatalysis
subject: gaseous phase
subject: CO2
subject: carbon dioxide
subject: CO2 reduction
subject: photosensitised homogeneous-heterogenerous system

description: Environment and energy security issues derived from the immoderate consumption of finite fossil fuels have promoted the discovery of environment friendly and sustainable energy conversion strategy. The burning of fossil fuels led to large amount output of carbon dioxide (CO2), which can either cause global warming if emitted to the atmosphere freely or become abundant feedstock if utilised properly. Photocatalytic CO2 reduction reaction can convert CO2 into more valuable carbonaceous molecules, such as carbon monoxide (CO), methane (CH4), formic acid (HCOOH), or even alcohols, by harnessing infinite solar energy, and this process will contribute to alleviate both above issues. In this process, catalysis materials play a key role, and reasonable designed reaction system can provide supportive platform for catalysts to achieve desirable performance. Numerous studies have focused on developing advanced catalysts and various configuration of CO2 reduction. However, current CO2 reduction system are still encountering with enormous challenges, including the competition of hydrogen evolution reaction (HER), further leap in activity, diversity of products and complexity of reaction mechanism and active sites. This thesis aims to improve the performance of traditional heterogenous photocatalysis in gaseous phase CO2 reduction by enhancing the light utilization and mass transfer efficiency and develop new photosensitised homogeneousheterogenerous system for enhanced selectivity and activity in CO2 reduction. Hierarchical hollow-shell nanomaterials have attracted numerous attentions in multidisciplinary research, especially in photocatalysis area, due to their unique structure and outstanding properties. However, current hard-templating synthesis methods are mainly capable of fabricating hierarchical hollow-shell structure composed with singular component and binary semiconductors, while show some drawbacks to synthesize more complicated compositions. Rational fabricating hierarchical hollow-shell structure composed with multinary semiconductor or semiconductors’ heterojunction remains challenging. In Chapter 2, the wide applicability of a unique one-pot hard-template method was discussed. With the strategy of fixing one component into templating agent (e.g. sucrose) at very beginning, followed by stepwise adsorption of other species, this method was capable of fabricating hierarchical hollow-shell structure composed of multinary semiconductor and semiconductors’ heterojunction. Besides, gaseous phase CO2 photoreduction reactions under high pressure condition were tested on synthesized hierarchical hollow-shell semiconductors’ heterojunction samples which achieved high selectivity and activity towards carbonaceous products. Highly selective photocatalytic CO2 reduction into CO and methane (CH4) by solidgas interface reaction mode is achieved under ambient pressure on elaborately designed hollow sphere based TiO2/SrTiO3 heterostructures. The synthesized three-dimensional hierarchical hollow multi-shelled TiO2/SrTiO3 spheres (HoMSs-TS) are an assembly of well-organized shells composed of interconnected anatase TiO2 and perovskite SrTiO3 nanoparticles (NPs). This hierarchical structure features multiple porous shells and subunits, which not only serve to maximize light utilization, but also facilitate mass transport during the gaseous phase photocatalytic process. Moreover, the heterojunctions between TiO2 and SrTiO3 provide synergistic enhancement of the charge transfer and separation process, which is equally critically important in the selective photocatalytic CO2 reduction reaction (CO2RR). These advantages empower the HoMSs-TS to effectively suppress hydrogen evolution and ease the selective production of CO and CH4. This work is described in Chapter 3 of this thesis. Photosensitized heterogeneous CO2 reduction (PHCR), especially the Ru-Co based PHCR system, has emerged as a promising visible-photocatalytic photocatalysis system for converting CO2 into value-added chemicals, however, challenged by the hydrogen evolution reaction (HER) induced low selectivity. Chapter 4 of this thesis reports a PHCR system that couples Ru(bpy)3^2+ homogeneous sensitizer with (001) faceted ultrathin LiCoO2 nanosheets heterogeneous photocatalyst with completely suppressed HER to yield 100% carbonaceous products. The experimental and theoretical studies reveal that the favored CO2 adsorption on the exposed Li on (001) faceted LiCoO2 surface is responsible for the suppressed HER. Despite recent progress in discovering various photocatalysts, realizing high performance in sensitised homo-hetero CO2 reduction and disclosing CO2 activation site remain challenging. In the work of Chapter 5, we report the synergistic effect of superficial Co-O nano layer and heterogenous Co metallic in enhancing PHCR system. The conductive heterogenous cobalt metallic core composed of face-centred cubic (fcc)cubic Co and hexagonal closest packed (hcp) Co was formed by heat treatment of Co3O4 in reductive atmosphere and can effectively improve the conductivity of photogenerated electron. Moreover, an in situ formed thin oxide shell on the surface provided active sites for CO2 reduction reaction. Owing to the synergistic effect, the performance in photosensitised CO2 reduction into CO is about 1.7 times comparing to that of Co3O4 nanosheets under the same reaction condition. This discovery provides a promising approach for the design of other materials for energy conversion. This thesis significantly contributes innovative knowledge in material science and CO2 reduction system through: (i) designing novel 3D structured heterogeneous photocatalyst with high photocatalytic CO2 reduction performance toward CO and CH4; (ii) developing gaseous phase CO2 reduction to suppress hydrogen evolution reaction (HER); (iii) reviewing the development of photosensitised homogeneous-heterogeneous CO2 reduction, and developing new approach to improve the selectivity by manipulating exposed ion as active site; (iv) probing the in situ formation of oxidised nanolayer around metallic core and investigating the synergetic effect of interface of oxidised layer and heterogenous metallic core in improving CO2 reduction performance in PHCR reaction system.