CO₂ reduction via photo- and photo-thermo coupled heterogeneous catalytic reaction

Abstract

CO₂ has been seen as a green-house gas mainly responsible for the global climate change. According to National Oceanic and Atmospheric Administration (NOAA), CO₂ emission since 1980 contributes to half of the CO₂ increases in the last 300 years and 25% of the CO₂ concentration rise is attributed to the emission since 2000. The CO₂ concentration is increasing faster than ever now. Instant and effective actions are urgently required to curb the trend in order to constrain the global temperature increase lower than 1.5 °C. Besides CO₂ capture and storage, CO₂ catalytic conversion to value-added chemicals is another important strategy to reduce its concentration in the atmosphere.

Solar light is one of the massive and widespread sustainable energy sources on the Earth. Using solar energy to drive CO₂ reduction is a promising and green way to reduce CO₂ concentration in atmosphere. The main challenges in practical implementation of the CO₂ conversion in large-scale are the relatively slow reaction rates and poor selectivity to more valuable products. The key to address these issues relies on better understanding of the reaction and photoexcitation mechanisms. This thesis focuses on building the correlation between the microstructure of catalysts, light induced excitation, reaction pathway, product selectivity and reaction rate.

The research work presented in this thesis starts from investigating the CO₂ photocatalytic reduction with H₂O over g-C₃N₄ (Chapter 3). A comprehensive optimisation of molecular and electronic structure of pristine g-C₃N₄₄ is conducted. The designed modification of co-doped K, B and N Vacancy (NV) enhances light absorption, excited-electron/hole separation, CO₂ adsorption, therefore the overall significantly increases the photocatalytic CO and CH₄ production rate by 527% and 161% comparing with the pristine g-C₃N₄. The reaction mechanism has been investigated via in-situ DRIFTS. An important correlation is found between activated CO₂* concentration on catalyst surface and reaction rate. The enhanced capability to activate the adsorbed CO₂* is the key guidance for future photocatalysts design.

Most of the current CO₂ photocatalytic reduction with H₂O can only produce CH₄ and CO. Therefore, Chapter 4 focuses on how to promote higher hydrocarbon production via CO₂ photocatalytic reduction with H₂O over Au/TiO₂-x with abundant oxygen vacancies (VO). The reaction performances are compared under UV and green light irradiation conditions. The UV light-driven CO₂ reduction shows primary product of CO; by comparison, the green light-driven CO₂ reduction produces 20% C₂H₆ and 80% CH₄. The electronic structure and microstructure of Au/TiO₂₂-x are investigated by a series of in-situ spectroscopy methods. The involvement of VO is found indispensable for this reaction. The different product selectivity under ultraviolet (UV) or green light irradiation is derived from the different excitation mechanism. The UV light bandgap excitation induces negatively charged Au that retards CO* adsorption; on the contrary, the green light plasmonic excitation causes positively charged Au that stabilises CO* adsorption. The stability of CO* key reaction intermediate is essential for higher hydrocarbon production.

The two chapters mentioned above have demonstrated the feasibility to significantly improve the photocatalytic CO₂ reduction rate by optimising the catalysts. However, the photocatalytic reaction rate is still far from comparable with traditional thermocatalytic CO₂ reduction. The H₂O has been proven as a less effective proton donor in Chapter 4 and the role of H₂O needs further investigations. Therefore, in the Chapter 5 & 6, CO₂ hydrogenation to CO, known as the reversed water gas shift reaction (RWGS reaction), is investigated under photo-thermo coupled catalytic reaction condition. The external light promoted thermocatalytic RWGS reaction performances are compared over two typical catalysts: Au/TiO₂ and Au/γ-Al₂O₃. The reducible TiO₂ and non-reducible γ-Al₂O₃ supports are fundamentally different on whether the plasmonic hot electrons transfer to the support. The CO₂ reduction rates are found significantly higher over Au/TiO₂ than Au/Al₂O₃ on either pure thermocatalytic or photo-thermo coupled catalytic reaction conditions. Moreover, the plasmonic enhancement efficiency is also superior on Au/TiO₂ over Au/A₂O₃.

In Chapter 5 & 6, the RWGS reaction mechanism in dark and photo-thermo coupled conditions are investigated via a series of in-situ spectroscopy and isotope kinetic analyses. The plasmonic enhancement mechanisms are also investigated via both experimental and theoretical approaches. The reaction mechanisms are proven to be quite different over Au/TiO₂ and Au/Al₂O₃. RWGS reaction follows a redox pathway with CO₂ self-dissociation at the VO site of Au/TiO₂. By comparison, the RWGS reaction mechanism over Au/Al₂O₃ is a mixture of formate and carboxyl reaction pathways. Accordingly, the plasmonic enhancement mechanisms are different too. In the case of Au/TiO₂, plasmonic hot electrons mainly facilitate the VO generation; while on Au/Al₂O₃, the formate pathway, especially the formate in bridged configuration, is preferentially promoted by plasmonic excitations.

Overall, this thesis has expanded and deepened the understandings of the photocatalytic and photo-thermo coupled catalytic CO₂ reductions. The mechanisms elucidated in this thesis can inspire future works on designing and developing more efficient and economically-feasible CO₂ utilisation to address climate change challenge.