A photon–phonon cascade catalytic process has been developed for the efficient oxidation of methane to formaldehyde (HCHO). This method achieves a high productivity of 401.5 μmol h⁻¹ (or 40,150 μmol g⁻¹ h⁻¹) and a selectivity of 90.4% at 150 °C using a ZnO catalyst decorated with single Ru atoms. The process involves photocatalytic conversion of methane and water to methyl hydroperoxide, followed by thermodecomposition to produce HCHO. Single Ru atoms act as electron acceptors, enhancing charge separation and promoting oxygen reduction in photocatalysis. This approach minimizes energy consumption and improves efficiency, offering a promising pathway for the sustainable transformation of light alkanes. Methane, a major component of shale gas and methane hydrate, is abundant on Earth. Converting methane into high-value chemicals and fuels through low-carbon processes is crucial for maximizing the economic value of fossil feedstocks and reducing greenhouse gas emissions. Formaldehyde is a key precursor in the production of industrial resins and plastics, and is also used in the textile and pharmaceutical industries. The current industrial process for HCHO production involves steam reforming of methane to produce synthesis gas, followed by methanol oxidation. However, this multi-step process is energy-intensive and not economically feasible for localized applications. Direct oxidation of methane to HCHO bypasses the synthesis gas and methanol oxidation steps, making the process more eco-friendly and efficient. However, the thermal activation of methane over metal-oxide-based catalysts requires high temperatures, leading to overoxidation of HCHO to CO and CO₂. Photocatalysis, using photons as an energy source, can activate and convert methane to oxygenates or hydrocarbons under mild conditions. Recent studies have shown that methane can be upgraded to HCHO via photocatalysis using various metal oxide semiconductors. However, the production rate of HCHO remains relatively low, and prolonged reaction times lead to overoxidation due to sluggish desorption of HCHO from the catalyst surface. To enhance the production rate of HCHO through photocatalysis, the introduction of phonon energy is reasonable due to the enhanced reaction kinetics. The first C–H bond dissociation is the rate-determining step in methane conversion, and thermal catalysis cannot overcome this energy barrier under mild conditions, while photocatalysis can readily do it thanks to the quantum effect of photons. However, photocatalysis can easily induce overoxidation of the valuable products to CO₂ due to the strong oxidation potential of the photoholes in a semiconductor if the reaction time is extended, whereas phonon energy can effectively tune the selectivity under mild conditions due to the fast desorption of the products at an elevated temperature. The ideal process is therefore a cascade process, in which photocatalysis initiates the first C–H dissociation, leading toA photon–phonon cascade catalytic process has been developed for the efficient oxidation of methane to formaldehyde (HCHO). This method achieves a high productivity of 401.5 μmol h⁻¹ (or 40,150 μmol g⁻¹ h⁻¹) and a selectivity of 90.4% at 150 °C using a ZnO catalyst decorated with single Ru atoms. The process involves photocatalytic conversion of methane and water to methyl hydroperoxide, followed by thermodecomposition to produce HCHO. Single Ru atoms act as electron acceptors, enhancing charge separation and promoting oxygen reduction in photocatalysis. This approach minimizes energy consumption and improves efficiency, offering a promising pathway for the sustainable transformation of light alkanes. Methane, a major component of shale gas and methane hydrate, is abundant on Earth. Converting methane into high-value chemicals and fuels through low-carbon processes is crucial for maximizing the economic value of fossil feedstocks and reducing greenhouse gas emissions. Formaldehyde is a key precursor in the production of industrial resins and plastics, and is also used in the textile and pharmaceutical industries. The current industrial process for HCHO production involves steam reforming of methane to produce synthesis gas, followed by methanol oxidation. However, this multi-step process is energy-intensive and not economically feasible for localized applications. Direct oxidation of methane to HCHO bypasses the synthesis gas and methanol oxidation steps, making the process more eco-friendly and efficient. However, the thermal activation of methane over metal-oxide-based catalysts requires high temperatures, leading to overoxidation of HCHO to CO and CO₂. Photocatalysis, using photons as an energy source, can activate and convert methane to oxygenates or hydrocarbons under mild conditions. Recent studies have shown that methane can be upgraded to HCHO via photocatalysis using various metal oxide semiconductors. However, the production rate of HCHO remains relatively low, and prolonged reaction times lead to overoxidation due to sluggish desorption of HCHO from the catalyst surface. To enhance the production rate of HCHO through photocatalysis, the introduction of phonon energy is reasonable due to the enhanced reaction kinetics. The first C–H bond dissociation is the rate-determining step in methane conversion, and thermal catalysis cannot overcome this energy barrier under mild conditions, while photocatalysis can readily do it thanks to the quantum effect of photons. However, photocatalysis can easily induce overoxidation of the valuable products to CO₂ due to the strong oxidation potential of the photoholes in a semiconductor if the reaction time is extended, whereas phonon energy can effectively tune the selectivity under mild conditions due to the fast desorption of the products at an elevated temperature. The ideal process is therefore a cascade process, in which photocatalysis initiates the first C–H dissociation, leading to