Insight on reaction pathways of photocatalytic methane conversion

Methane, one of the primary components of natural gas, is an excellent carbon and hydrogen source with low cost and natural abundance. It serves as an ideal feedstock for the production of high-value-added chemicals and fuels. However, its symmetrical tetrahedral structure presents intrinsic challenges for activation and conversion under mild conditions. Unlike the traditional high-temperature processes of methane reforming (700-1,100 °C) used in industry, the photocatalytic conversion of methane into high-value chemicals under mild conditions has attracted significant attention recently. Such a nature-mimicking approach leads to energy savings, reduces conversion costs, and decreases carbon emissions. However, current research on photocatalytic methane conversion is still far from scaling up, with diverse reaction mechanisms across different reaction systems, and there is a lack of a unified summary regarding the underlying mechanisms. Therefore, it is crucial to summarize various reported mechanisms and pathways, which are essential for guiding the design and optimization of catalysts and reaction systems. Herein, we review the most likely conversion pathways and common methods for photocatalytic methane conversion in different systems. This review categorizes the reaction mechanisms according to four common pathways in photocatalytic methane conversion: partial oxidation, coupling (including oxidative and non-oxidative coupling), functionalization, and reforming. Finally, it offers perspectives on the outlook of photocatalytic methane conversion from the angles of mechanistic research, catalyst design, and practical applications, providing insights into the fundamental aspects of this field.