Design and Synthesis of Catalysts for Photo-thermal CO₂ Hydrogenation

Abstract

For the excessive CO₂ emissions of global environmental issues, reducing CO₂ to renewable fuels for carbon neutrality and energy storage is a promising method. In response to the growing demand for sustainable carbon-cycle utilization, advanced technologies including photocatalytic, electrocatalytic, and thermocatalytic CO₂ hydrogenation have been developed. Nevertheless, these traditional techniques are typically confronted with several bottlenecks: low light-utilization efficiency and suboptimal activity, high potential barriers and unsatisfactory product selectivity, and harsh reaction conditions (eg. high temperature/pressure).

In recent years, photo-thermal coupling catalysis, leveraging the mutual promotion of photo-generated carriers and heat-induced effects, has emerged as a promising way to address relevant issues. At present, it has been effectively applied in the synthesis of CH₄, CH₃OH, CO, and light olefins. The generation of photo-generated carriers can modify the electronic structure on the catalyst surface, rendering the adsorption and activation of carbon dioxide and hydrogen more facile. Moreover, heat can further facilitate the separation and transport of photo-generated carriers, reducing their recombination probability and thereby enhancing the photocatalytic efficiency. Simultaneously, the photo-thermal synergy might also alter the reaction pathways and intermediates, enabling the reaction to progress in a more advantageous direction. Nevertheless, the equilibrium between selectivity and activity, the design of catalysts for active sites and light-absorbing units, stability, and reaction kinetics remain formidable challenges in photo-thermal CO₂ hydrogenation.

This thesis is centered around the design and synthesis of a novel catalyst for photo-thermal CO₂ hydrogenation. In-depth investigations are carried out not only on the effects of crystal facets but also on the coupling mechanism, especially the regulatory effect of light on the product. This thesis is structured into five chapters:

Chapter 1: Presents a comprehensive review of the global advancements in photo-thermal CO₂ hydrogenation. It elaborates on diverse types of catalysts and device designs, while also engaging in critical reflection on the existing research outcomes and exploration of underlying scientific issues.

Chapter 2: A detailed description of the experimental procedures is provided, including the instruments, reagents, experimental facilities, characterization methods, calculation formulas, etc.

Chapter 3: Through facet engineering, a Ru/SrTiO₃ nanosheet catalyst (Ru/STO) exposing the {110} facet is designed and successfully synthesized. Under mild conditions (280^oC, 0.1 MPa) in the flowing CO₂, the introduction of light irradiation doubled the reaction rate while maintaining approximately 100% CH₄ selectivity. The {110} facet terminated with Sr-O-Ti is conducive to activating the CO₂ molecule while firmly anchoring the CO intermediate, ensuring high CH₄ selectivity. Furthermore, the local surface plasmon resonance (LSPR) effect of Ru nanoparticles facilitates the Hs adatom diffusion at the interface and greatly improves the catalytic activity.

Chapter 4: Upon the introduction of the Cu element, the regulatory effect of light on product selectivity was remarkably discerned. Through a series of comprehensive analyses including activity comparison, activation energy calculation, in-situ characterization, and DFT calculation simulations, it was firmly established that during the photo-thermal CO₂ hydrogenation process, aside from the anchoring effect of RuCu-x alloy on the intermediate product CO, light played a crucial role. It suppressed the proportion of CO in the final product by supplying more hot electrons for the deep hydrogenation of CO. Subsequently, this finding was generalized to a broader range of transition metals (Fe, Co, Ni) to explore its universality.

Chapter 5: Finally, a comprehensive summary and outlook of the entire thesis are presented, and the unresolved issues and future development directions were identified in photo-thermal CO₂ hydrogenation.

Date of Award	1 Aug 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yung-kang PENG (Supervisor) & Zhanxi FAN (Supervisor)