Core–Shell MIL-125-NH₂@FeOOH Nanocomposites for Highly Selective Photocatalytic Oxidation of Methane to Formaldehyde in Water Vapor

Formaldehyde (HCHO), a crucial industrial chemical, finds extensive applications across diverse sectors, including household products, commercial materials, aviation, and medical supplies. Methane (CH₄), as an abundant C1 resource, presents a promising feedstock for HCHO synthesis. However, the direct conversion of CH₄ to HCHO remains challenging due to its inherent chemical inertness, characterized by low polarizability and high C-H bond dissociation energy (439 kJ mol^-1), coupled with the high reactivity of intermediate products. The development of efficient strategies for selective CH₄ oxidation to high-value HCHO under mild conditions is therefore of significant practical importance. In this study, we developed a series of MIL-125-NH₂@FeOOH-<i>x</i> heterostructured photocatalysts (FM-<i>x</i>) through the controlled deposition of FeOOH nanoparticles on MIL-125-NH₂ surfaces. Comprehensive characterization and photocatalytic evaluations reveal that the optimized FM-1 catalyst facilitates in situ H₂O₂ generation and subsequent decomposition into hydroxyl radicals (^•OH), enabling efficient CH₄ photooxidation. Remarkably, the FM-1 catalyst achieves an exceptional HCHO production rate of 197.79 μmol·g_cat^-1 with >99.99% selectivity in water vapor, significantly outperforming both pristine FeOOH and MIL-125-NH₂ components. This work presents a promising photocatalytic system for selective CH₄ conversion, offering new insights into the design of efficient catalysts for C1 chemistry.