Methane (CH4), a key component of combustible ice, is a plentiful yet largely underutilized ocean resource. Its extreme inertness and the complexity of its storage and transportation have severely hindered its use. Here, we present a tandem flow system that produces freshwater through seawater desalination, while the resulting concentrated seawater is rich in chlorine ions and serves as a source for CH4 chlorination over a floatable photocatalyst with a three-phase interface. The key lies in tailoring the interfacial acid-base microenvironment and electronic structure of Pt-TiO2 photocatalyst to steer the radical processes toward selective methane chlorination. ·OH radicals mediate C-H activation to generate ·CH3 radicals, which subsequently couple efficiently with interfacial ·Cl radicals to yield CH3Cl. The introduction of trace Lewis acids (e.g. Al3+) further enhances selectivity by regulating the local proton balance, suppressing OH- accumulation, and stabilizing key radical intermediates. Our system achieves 98.1% CH3Cl selectivity with production rates of 2.9 mmol g-1 h-1 in AlCl3 solution and 79.5 mmol m-2 h-1 under flow operation, demonstrating long-term stability over 15 days. This work establishes a general paradigm for controlling aqueous radical chemistry under mild conditions, and paves the way for the practical exploitation of offshore ocean resources.
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