Photocatalytic Methane Oxidation to Carbonyl Products.

Direct photocatalytic oxidation of methane (CH4) to value-added carbonyl compounds, such as formaldehyde (HCHO), formic acid (HCOOH), acetaldehyde (CH3CHO), and acetic acid (CH3COOH), represents an attractive and promising alternative to conventional indirect routes via syngas, which suffer from high energy consumption and carbon emissions. By utilizing light energy to activate the C─H bond under mild conditions, photocatalysis effectively prevents the overoxidation of the target compound, thereby alleviating the long-standing trade-off between activity and selectivity imposed by thermodynamic constraints. A deep understanding of the reaction mechanism underlying photocatalytic CH4 conversion to carbonyl compounds is therefore a prerequisite for designing high-performance photocatalytic systems; yet, this topic remains unexplored to date. In this review, we systematically discuss diverse reaction pathways from CH4 to C1 and C2 carbonyl compounds and summarize a comprehensive toolbox for mechanistic investigation, including basic characterizations, in situ spectroscopies, necessary control experiments, probing and trapping experiments, isotope labelling, and theoretical calculations. We then assess recent advances in catalytic systems and identify the key factors governing carbonyl formation, selectivity, and productivity. Finally, we highlight the central challenges that remain for practical implementation, particularly regarding catalyst performance, rational reaction system design, efficient product separation, and possible cascade reactions.