Li, Xiyi; (2022) Photocatalytic conversion of methane to C2 Products in a flow reactor. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The predicted substantial reserve of methane hydrate and shale gas, far beyond the sum of other fossil fuels, indicates an extremely attractive while challenging chemical synthesis process in which methane, instead of crude oil can be used as a building block for diverse chemical synthesis in a potentially low-carbon process. Among various direct methane conversion to value-added chemicals processes, photocatalytic oxidative coupling of two methane molecules to produce C2 products (C2H6/C2H4) is one of the most general and highly profitable but challenge processes for long chain chemical synthesis. However, due to the highly stable methane molecules and the more reactive C2 products, there is an obvious dilemma: an extreme condition (e.g., high energy or strong oxidants) required to activate the highly stable C-H bonds in methane, while a mild condition needed to avoid the overoxidation of C2 products. In the project, a flow system for photocatalytic oxidative coupling of methane was designed for the first time, which allows the manipulation of residence time and the CH4/O2 ratio for precise evaluation of the potential photocatalysts. The robust and classic anatase TiO2 was selected as a basic semiconductor for the investigation of methane activation. The most general electron acceptor in photocatalysis, Pt nanoparticles, and the widely used species for methane activation in thermocatalysis, CuOx clusters were introduced on TiO2 to work synergistically. The optimised sample Cu0.1Pt0.5/TiO2 shows the highest yield of C2 product of 6.8 μmol h-1 at a space velocity of 24000 ml g-1 h-1, more than twice the sum of the activity of Pt/TiO2 (1.07 μmol h-1) and Cu/TiO2 (1.9 μmol h-1), it was also the highest among photocatalytic methane conversion under atmospheric pressure when it was published. High C2 selectivity of 60% is also comparable to that attained by conventional high-temperature (>943 K) thermal catalysis. Characterisation data confirms that Pt acts as an electron acceptor to promote the charge separation, while holes are recommended to be accepted by CuOx to avoid overoxidation of as-formed C2 products. However, the yield rate and apparent quantum efficiency (AQE) are still low and the short stability test period is another concern. Therefore, a series of noble metals were loaded on TiO2 to form new photocatalysts, which usually act as charge sink to promote charge transfer in photocatalysis and show unique catalytic performance for C-H bond activation in traditional catalysis. Among them, Pd loading shows the exceptional activity and then a series of non-noble transition metals as the second component was introduced to form nanoalloy to assemble multi-function to enhance the activity and stability. The optimised sample PdCu/TiO2 shows an unprecedented activity, e.g., 116 h-1 of TOF and 12642 of TON even under a high space velocity of 342000 mL gcat-1 h-1 and >110 hours stability with the high C2 selectivity (75%) in a flow reactor operated at room temperature. In particular, the highest methane conversion rate of 2480 μmol g-1 h-1 to C2 products operated at room temperature has been achieved, 20 times higher than the results reported before. The highest AQE of 8.4% is also obtained among all ambient photocatalytic methane conversion to C2 processes, indicating it is an energy-efficient process. In-situ EPR, XPS and DFT calculations indicated that the unprecedented activity and stability are due to synergetic effect between Pd and Cu in the nanoalloy. The photoholes from TiO2 can be transferred to Pd, which lowers the oxidative potential to selectively abstract the pre-soften C-H bond in methane to form C2 products. While the introduction of metallic Cu can weaken the adsorption of C2 products, avoiding the further consecutive reaction for coke formation. Despite of the great improvement of activity and stability, the C2 yield rate and C2 selectivity is relatively moderate. The limitations of this flow reactor have also not been carefully considered, such as the relationship between activity and catalyst mass, the light utilisation efficiency and the thickness of membrane. In addition, the fabrication procedure of the photocatalysts is time-consuming and difficult to produce at large scale, impeding its further practical application. Therefore, a Au/TiO2 photocatalyst produced via a facile and rapid (60 seconds) sputter method was developed. After optimisation of both photocatalyst and reaction system, a very high C2 yield rate of 468 μmol h-1 (23.4 mmol g-1 h-1) with a decent C2 selectivity of ca. 90% can be obtained. The highest AQE of ca. 10% has also been achieved among all the reported methane conversion at ambient conditions. The charge transfer pathway is proposed based on the in situ XPS and transient absorption spectroscopy (TAS) measurements. The surface chemistry reaction between substrates and catalysts is investigated by in situ EPR and in situ DRIFTS. It has been found that the metallic Au nanoparticles work as a hole acceptor to efficiently extract the photoholes on TiO2 and drive the selective oxidation of methane to methyl radicals for further C-C coupling.

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