Wang, Jingyi; (2025) Engineering metal catalytic sites for efficient photocatalytic greenhouse gas conversion. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This PhD work focuses on developing efficient photocatalysts to convert CO2 and CH4, two major greenhouse gases, through their respective reaction pathways: CO2 reduction reaction (CO2RR) and non-oxidative coupling of methane (NOCM). To achieve high photocatalytic performance, the engineering of metal catalytic sites in photocatalyst design plays a pivotal role in the effective activation and conversion of CO2 and CH4 molecules. To achieve efficient CO2 photoreduction, a 3D cage-like ligand, cucurbit[7]uril (CB[7]), is utilized to anchor Ni atomic catalytic sites, leveraging its intrinsic cavity structure for CO2 enrichment and its carbonyl-fringed portals for effective metal coordination. The novel molecular catalyst, cucurbit[7]uril-nickel complex (CB[7]-Ni), not only possesses the homogeneous nature of a molecular catalyst, allowing efficient charge transfer between photosensitiser and active centre, but also enhances CO2 adsorption via the unique hydrophobic cage on ligand. The Ni active sites in CB[7]-Ni achieve the highest CO selectivity (99%) and an exceptional CO yield rate (75 µmol·h-1) under visible light irradiation for CO2 photoreduction, outperforming other common non-noble metal species. Moreover, CB[7]-Ni shows the highest CO yield rate with a high CO selectivity among the reported homogeneous and heterogeneous photocatalytic systems for CO2-to-CO reduction. A new and facile tandem isotopic reaction is further developed to rigorously confirm the carbon source of the as-formed CO product. Following the comprehensive investigation of CB[7]-Ni complex in CO2 photoreduction, a catalytic membrane is developed via the heterogenization strategy, aiming at enhancing the potential applicability of the efficient homogeneous catalyst. Due to its structural stability under light irradiation and its role in facilitating CO2 conversion through local proton activity, the Nafion membrane is selected as the support for the heterogenization of CB[7]-Ni catalyst (CB[7]-Ni/Nafion) for CO2 photoreduction. Benefiting from the Ni catalytic sites and CO2 enrichment effect of the CB[7] cage, CB[7]-Ni/Nafion exhibits a CO yield that is 20 times higher than that of the pristine Nafion membrane under visible light irradiation. Comprehensive characterizations confirm the effective integration of CB[7]-Ni onto Nafion membrane. Despite the necessity for further enhancements in the reusability of the photocatalytic membrane, this study presents a promising and novel approach for the heterogeneous engineering of molecular catalysts using membrane support. Catalyst activity and stability have been the challenges in efficient photocatalytic NOCM. Bimetallic catalysts thus have been widely studied, aiming at enhancing two key half-reactions: the selective activation and coupling of CH4, and the efficient H+ reduction to produce hydrogen. Au has been reported as a photohole transport pathway, enhancing CH4 adsorption, C-H activation and C-C coupling. Meanwhile, Pt is selected as the other co-catalyst for reductive half reaction due to its role of photoelectron sink for proton reduction in water splitting. Harnessing the synergistic effects of dual metal cocatalysts loaded onto the classic TiO2, known for its high oxidation potential, Pt-Au/TiO2 exhibits unprecedented production rates of H2 (22.3 μmol·h−1) and C2+ hydrocarbons (21.2 μmol·h−1), with high C2+ selectivity (98.8%) and remarkable catalyst stability (> 215 h) under 365 nm LED irradiation. Comprehensive characterizations reveal the synergistic charge transport pathways of Pt and Au in NOCM, with proposed spillover mechanisms involving proton and methyl intermediates. This breakthrough in effectively balancing activity, selectivity, and stability highlights the potential of dual-metal catalytic site strategies for advancing photocatalytic flow system in NOCM.

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