Wang, Chao; (2024) Selective oxidation of methane using oxide photocatalysts in a pressurised flow reactor. Doctoral thesis (Ph.D), UCL (University College London).

Text Thesis Chao Wang.pdf - Published Version Access restricted to UCL open access staff until 1 June 2025. Download (9MB)

Abstract

Large reserves of natural gas and shale gas, especially those in remote areas, have raised incentives for the on-site and large-scale conversion of methane to high-value chemicals. However, the inert nature of methane molecules originating from the high C-H bond energy and symmetric structure makes its economic conversion very challenging. Among all methane conversion processes, the oxidation of methane using oxygen gas to various products are exothermic reactions, making efficient conversion of methane potentially more achievable in this approach. Traditional thermocatalysis uses high temperatures to activate methane, which is a high-cost and energy-intensive process. Photocatalysis uses the energy of photons instead of heat to drive reactions under mild conditions and can potentially offer a green and economical methane oxidation process. In this project, a pressurised flow reaction system for photocatalytic methane oxidation is designed and assembled. With the current setup, the flow rate and ratio of reactants, reaction pressure, temperature, light source, and irradiation intensities can be readily and reproducibly adjusted during photocatalytic methane oxidation. Moreover, online timely detection of all products is achieved. Firstly, TiO2 is used as a photocatalyst for methane oxidation. To improve the photon efficiency, Ag is loaded on TiO2 as a co-catalyst. AgBr is further loaded on Ag/TiO2 to improve the selectivity of C2+ hydrocarbons and reduce overoxidation. As a result, the optimised Ag-AgBr/TiO2 photocatalyst displays a high C2H6 yield of 35.4 μmol h-1 without external heating and under 6 bar pressure. The C2H6 selectivity ranges from 69% to 87% depending on the pressures applied. An apparent quantum efficiency (AQE) of 3% at 365 nm is achieved. Detailed characterisation proved that Ag acts as an electron acceptor and promotes oxygen reduction reaction, while AgBr suppresses overoxidation and improves the C2+ selectivity by reducing the oxidation potential of photoholes. The synergy of Ag and AgBr is proved to also work on ZnO for photocatalytic oxidative coupling of methane. The methane oxidation performance achieved over Ag-AgBr/TiO2 is still moderate in terms of both product yield and selectivity compared with the thermocatalysis. The light-sensitive AgBr also causes a stability issue for photocatalysis. Moreover, the investigated TiO2 and ZnO are only active under UV light (λ<400 nm). The development of an efficient, selective, and robust methane oxidation photocatalyst that works under visible light is highly desirable. CeO2, a semiconductor with an absorption edge of 400-500 nm, is selected as the photocatalyst for methane oxidation. Au is loaded on CeO2 as a co-catalyst. A high ethane yield of 755 μmol h-1 (15,100 μmol g 1 h 1) and a C2H6 selectivity of 93% are obtained under the optimised reaction condition, corresponding to an AQE of 12% at 365 nm. Moreover, the high activity can be maintained without noticeable decay for at least 120 h. More importantly, the methane oxidation performance measured under visible light is higher than most of the reported photocatalysts tested under UV light. A series of spectroscopy investigations prove that Au serves as a hole acceptor and promotes charge separation. Moreover, ethane oxidation reaction and in situ DRIFTS confirm that Au improves the C2H6 selectivity by inhibiting overoxidation and promoting the C-C coupling reaction. To understand the origin of the high photocatalytic methane oxidation performance of CeO2-based photocatalysts. A series of CeO2 nanocrystals including nanocubes (CeO2-C), nanorods (CeO2-R), and nanopolyhedrons (CeO2-P) with exposed {001}, {101}, and {111} facets, respectively, are fabricated. Considering methane is also a potent greenhouse gas and a considerable amount of methane has been leaked into the atmosphere during the production, transportation, and consumption of natural gas, the as-prepared CeO2 nanocrystals are tested for photocatalytic deep oxidation of methane to mitigate its environmental effects. Total oxidation of methane is an ideal reaction to test the intrinsic activity of CeO2 for methane activation as CO2 is the sole product when bare CeO2 is applied as the photocatalyst. Among the three CeO2 nanocrystals, CeO2-P displays the highest methane oxidation performance, while the lowest methane conversion is achieved over CeO2-C. After loading 0.5 wt. % of Pt onto CeO2-P, a one-pass methane conversion of 95% and CO2 selectivity of 99% have been achieved. This high performance could be maintained for at least 100 h. In a methane-rich atmosphere, a high AQE of 36.5% is achieved at 365 nm. In situ photo-induced visible absorption spectroscopy suggests that CeO2-P with exposed {111} facets shows superior redox capabilities for both oxygen and methane activation. In situ DRIFTS provides evidence that the conversion of intermediate species over CeO2 P is much faster than that that over CeO2-C and CeO2-R. The activation energy of CeO2-P for methane oxidation is calculated to be 10.2 kJ mol-1, which is significantly lower than that of CeO2-C (19.5 kJ mol-1) and CeO2-R (17.8 kJ mol-1).

Download activity - last month

Export as JSONXMLCSV

Download activity - last 12 months

Export as CSVXMLJSON

Downloads by country - last 12 months

Export as XMLJSONCSV

Archive Staff Only

View Item