SiO₂@V₂O₅@Al₂O₃ core–shell catalysts with high activity and stability for methane oxidation to formaldehyde

The stable tetrahedral geometry and high C–H bond dissociation energy of methane complicate its direct catalytic conversion; for example, the selective oxidation of methane to formaldehyde, which avoids the production of carbon dioxide by full oxidation and is therefore important for the versatile utilization of natural gas, is still viewed as challenging. Here, we utilize hydrothermal synthesis followed by atomic layer deposition (ALD) to prepare an efficient and thermally stable catalyst based on novel SiO₂@V₂O₅@Al₂O₃ core@shell nanostructures, showing that the thickness of Al₂O₃ shells over SiO₂@V₂O₅ cores can be tuned by controlling the number of ALD cycles. Catalytic methane oxidation experiments performed in a flow reactor at 600 °C demonstrate that SiO₂@V₂O₅@Al₂O₃ nanostructures obtained after 50 ALD cycles exhibit the best catalytic activity (methane conversion = 22.2%; formaldehyde selectivity = 57.8%) and outperform all previously reported vanadium-based catalysts at 600 °C. The prepared catalysts are subjected to in-depth characterization, which reveals that their Al₂O₃ shell provides new surfaces for the generation of highly disperse T_d monomeric species with a V–O–Al bond by promoting interactions between Al₂O₃ and V₂O₅ nanoparticles during ALD. Moreover, the surface Al₂O₃ shell is found not only to protect V₂O₅ nanoparticles against sintering at 600 °C, but also to anchor the produced T_d monomeric vanadium species responsible for the high catalytic performance.